Publications Rainer BLATT

2024

Articles

• M. K. Joshi, M. Guevara-Bertsch, F. Kranzl, R. Blatt, C. F. Roos Characterization of ion-trap-induced ac magnetic fields, Phys. Rev. A 110 63101 (2024-12-01), http://dx.doi.org/10.1103/PhysRevA.110.063101 doi:10.1103/PhysRevA.110.063101 (ID: 721258) Toggle Abstract
  The oscillating magnetic field produced by unbalanced currents in radio-frequency ion traps induces transition frequency shifts and sideband transitions that can be harmful to precision spectroscopy experiments. Here, we describe a methodology, based on two-photon spectroscopy, for determining both the strength and direction of rf-induced magnetic fields without modifying any DC magnetic bias field or changing any trap RF power. The technique is readily applicable to any trapped-ion experiment featuring narrow linewidth transitions.
• L. Postler, F. Butt, I. Pogorelov, C. Marciniak, S. Heußen, R. Blatt, P. Schindler, M. Rispler, M. Müller, T. Monz Demonstration of fault-tolerant Steane quantum error correction, PRX Quantum 5 030326-19 (2024-08-07), http://dx.doi.org/10.1103/PRXQuantum.5.030326 doi:10.1103/PRXQuantum.5.030326 (ID: 721173) Toggle Abstract
  Encoding information redundantly using quantum error-correcting (QEC) codes allows one to overcome the inherent sensitivity to noise in quantum computers to ultimately achieve large-scale quantum computation. The Steane QEC method involves preparing an auxiliary logical qubit of the same QEC code used for the data<br /> register. The data and auxiliary registers are then coupled with a logical CNOT gate, enabling a measurement of the auxiliary register to reveal the error syndrome. This study presents the implementation of multiple rounds<br /> of fault-tolerant Steane QEC on a trapped-ion quantum computer. Various QEC codes are employed, and the<br /> results are compared to a previous experimental approach utilizing flag qubits. Our experimental findings show improved logical fidelities for Steane QEC. This establishes experimental Steane QEC as a competitive paradigm for fault-tolerant quantum computing.
• L. K. Joshi, J. Franke, A. Rath, F. Ares, S. Murciano, F. Kranzl, R. Blatt, P. Zoller, B. Vermersch, P. Calabrese, C. F. Roos, M. K. Joshi Observing the quantum Mpemba effect in quantum simulations, PRL 133 10402 (2024-07-01), http://dx.doi.org/10.1103/PhysRevLett.133.010402 doi:10.1103/PhysRevLett.133.010402 (ID: 721174) Toggle Abstract
  The non-equilibrium physics of many-body quantum systems harbors various unconventional phenomena. In this study, we experimentally investigate one of the most puzzling of these phenomena— the quantum Mpemba effect, where a tilted ferromagnet restores its symmetry more rapidly when it is farther from the symmetric state compared to when it is closer. We present the first experimental evidence of the occurrence of this effect in a trapped-ion quantum simulator. The symmetry breaking and restoration are monitored through entanglement asymmetry, probed via randomized measurements, and post-processed using the classical shadows technique. Our findings are further substantiated by measuring the Frobenius distance between the experimental state and the stationary thermal symmetric theoretical state, offering direct evidence of subsystem thermalization.
• H. Hainzer, D. Kiesenhofer, T. Ollikainen, M. Bock, F. Kranzl, M. K. Joshi, G. Yoeli, R. Blatt, T. Gefen, C. F. Roos Correlation Spectroscopy with Multiqubit-Enhanced Phase Estimation, Phys. Rev. X 14 24 (2024-02-29), http://dx.doi.org/10.1103/PhysRevX.14.011033 doi:10.1103/PhysRevX.14.011033 (ID: 721249) Toggle Abstract
  Ramsey interferometry is a widely used tool for precisely measuring transition frequencies between two energy levels of a quantum system, with applications in time keeping, precision spectroscopy, quantum optics, and quantum information. Often, the coherence time of the quantum system surpasses the one of the<br /> oscillator probing the system, thereby limiting the interrogation time and associated spectral resolution.<br /> Correlation spectroscopy overcomes this limitation by probing two quantum systems with the same noisy<br /> oscillator for a measurement of their transition frequency difference; this technique has enabled very<br /> precise comparisons of atomic clocks. Here, we extend correlation spectroscopy to the case of multiple<br /> quantum systems undergoing strong correlated dephasing. We model Ramsey correlation spectroscopy<br /> with N particles as a multiparameter phase estimation problem and demonstrate that multiparticle correlations can assist in reducing the measurement uncertainties even in the absence of entanglement. We<br /> derive precision limits and optimal sensing techniques for this problem and compare the performance of<br /> probe states and measurement with and without entanglement. Using one- and two-dimensional ion<br /> Coulomb crystals with up to 91 qubits, we experimentally demonstrate the advantage of measuring multiparticle correlations for reducing phase uncertainties and apply correlation spectroscopy to measure ion-ion distances, transition frequency shifts, laser-ion detunings, and path-length fluctuations. Our method can be straightforwardly implemented in experimental setups with globally coherent qubit control and<br /> qubit-resolved single-shot readout and is, thus, applicable to other physical systems such as neutral atoms in tweezer arrays.
• R. Stricker, J. Carrasco, M. Ringbauer, L. Postler, M. Meth, C. Edmunds, P. Schindler, R. Blatt, P. Zoller, B. Kraus, T. Monz Towards experimental classical verification of quantum computation, Quantum Sci. Technol. 9 (2024-02-26), http://dx.doi.org/10.1088/2058-9565/ad2986 doi:10.1088/2058-9565/ad2986 (ID: 720821) Toggle Abstract
  With today's quantum processors venturing into regimes beyond the capabilities of classical devices [1-3], we face the challenge to verify that these devices perform as intended, even when we cannot check their results on classical computers [4,5]. In a recent breakthrough in computer science [6-8], a protocol was developed that allows the verification of the output of a computation performed by an untrusted quantum device based only on classical resources. Here, we follow these ideas, and demonstrate in a first, proof-of-principle experiment a verification protocol using only classical means on a small trapped-ion quantum processor. We contrast this to verification protocols, which require trust and detailed hardware knowledge, as in gate-level benchmarking [9], or additional quantum resources in case we do not have access to or trust in the device to be tested [5]. While our experimental demonstration uses a simplified version [10] of Mahadev's protocol [6] we demonstrate the necessary steps for verifying fully untrusted devices. A scaled-up version of our protocol will allow for classical verification, requiring no hardware access or detailed knowledge of the tested device. Its security relies on post-quantum secure trapdoor functions within an interactive proof [11]. The conceptually straightforward, but technologically challenging scaled-up version of the interactive proofs, considered here, can be used for a variety of additional tasks such as verifying quantum advantage [8], generating [12] and certifying quantum randomness [7], or composable remote state preparation [13].

Preprints

• D. Miller, K. Levi, L. Postler, A. Steiner, L. Bittel, G. A. White, Y. Tang, E. J. Kuehnke, A. A. Mele, S. Khatri, L. Leone, J. Carrasco, C. Marciniak, I. Pogorelov, M. Guevara-Bertsch, R. Freund, R. Blatt, P. Schindler, T. Monz, M. Ringbauer, J. Eisert Experimental measurement and a physical interpretation of quantum shadow enumerators, (2024-08-29), http://dx.doi.org/10.48550/arXiv.2408.16914 doi:10.48550/arXiv.2408.16914 (ID: 721299) Toggle Abstract
  Throughout its history, the theory of quantum error correction has heavily benefited from translating classical concepts into the quantum setting. In particular, classical notions of weight enumerators, which relate to the performance of an error-correcting code, and MacWilliams' identity, which helps to compute enumerators, have been generalized to the quantum case. In this work, we establish a distinct relationship between the theoretical machinery of quantum weight enumerators and a seemingly unrelated physics experiment: we prove that Rains' quantum shadow enumerators - a powerful mathematical tool - arise as probabilities of observing fixed numbers of triplets in a Bell sampling experiment. This insight allows us to develop here a rigorous framework for the direct measurement of quantum weight enumerators, thus enabling experimental and theoretical studies of the entanglement structure of any quantum error-correcting code or state under investigation. On top of that, we derive concrete sample complexity bounds and physically-motivated robustness guarantees against unavoidable experimental imperfections. Finally, we experimentally demonstrate the possibility of directly measuring weight enumerators on a trapped-ion quantum computer. Our experimental findings are in good agreement with theoretical predictions and illuminate how entanglement theory and quantum error correction can cross-fertilize each other once Bell sampling experiments are combined with the theoretical machinery of quantum weight enumerators.
• C. Edmunds, E. Rico, I. Arrazola, G. Brennen, M. Meth, R. Blatt, M. Ringbauer Constructing the spin-1 Haldane phase on a qudit quantum processor, (2024-08-08), http://dx.doi.org/10.48550/arXiv.2408.04702 doi:10.48550/arXiv.2408.04702 (ID: 721300) Toggle Abstract
  Symmetry-protected topological phases have fundamentally changed our understanding of quantum matter. An archetypal example of such a quantum phase of matter is the Haldane phase, containing the spin-1 Heisenberg chain. The intrinsic quantum nature of such phases, however, often makes it challenging to study them using classical means. Here, we use trapped-ion qutrits to natively engineer spin-1 chains within the Haldane phase. Using a scalable, deterministic procedure to prepare the Affleck-Kennedy-Lieb-Tasaki (AKLT) state within the Haldane phase, we study the topological features of this system on a qudit quantum processor. Notably, we verify the long-range string order of the state, despite its short-range correlations, and observe spin fractionalization of the physical spin-1 particles into effective qubits at the chain edges, a defining feature of this system. The native realization of Haldane physics on a qudit quantum processor and the scalable preparation procedures open the door to the efficient exploration of a wide range of systems beyond spin-1/2.
• M. Valentini, M. van Mourik, F. Butt, J. Wahl, M. Dietl, M. Pfeifer, F. Anmasser, Y. Colombe, C. Rössler, P. Holz, R. Blatt, M. Müller, T. Monz, P. Schindler Demonstration of two-dimensional connectivity for a scalable error-corrected ion-trap quantum processor architecture, (2024-06-04), arXiv:2406.02406 arXiv:2406.02406 (ID: 721261) Toggle Abstract
  A major hurdle for building a large-scale quantum computer is to scale up the number of qubits while maintaining connectivity between them. In trapped-ion devices, this connectivity can be provided by physically moving subregisters consisting of a few ions across the processor. The topology of the connectivity is given by the layout of the ion trap where one-dimensional and two-dimensional arrangements are possible. Here, we focus on an architecture based on a rectangular two-dimensional lattice, where each lattice site contains a subregister with a linear string of ions. We refer to this architecture as the Quantum Spring Array (QSA). Subregisters placed in neighboring lattice sites can be coupled by bringing the respective ion strings close to each other while avoiding merging them into a single trapping potential. Control of the separation of subregisters along one axis of the lattice, known as the axial direction, uses quasi-static voltages, while the second axis, the radial, requires control of radio frequency signals. In this work, we investigate key elements of the 2D lattice quantum computation architecture along both axes: We show that the coupling rate between neighboring lattice sites increases with the number of ions per site and the motion of the coupled system can be resilient to noise. The coherence of the coupling is assessed, and an entangled state of qubits in separate trapping regions along the radial axis is demonstrated. Moreover, we demonstrate control over radio frequency signals to adjust radial separation between strings, and thus tune their coupling rate. We further map the 2D lattice architecture to code primitives for fault-tolerant quantum error correction, providing a step towards a quantum processor architecture that is optimized for large-scale fault-tolerant operation.
• Y. Weiser, T. Faorlin, L. Panzl, T. Lafenthaler, L. Dania, D. S. Bykov, T. Monz, R. Blatt, G. Cerchiari Back action suppression for levitated dipolar scatterers, (2024-02-07), arXiv:2402.04802 arXiv:2402.04802 (ID: 721241) Toggle Abstract
  Levitated dipolar scatterers exhibit exceptional performance as optomechanical systems for observing quantum mechanics at the mesoscopic scale. However, their tendency to scatter light in almost any direction poses experimental challenges, in particular limiting light collection efficiencies and, consequently, the information extractable from the system. In this article, we present a setup designed to enhance the information gleaned from optomechanical measurements by constraining the back action to a specific spatial direction. This approach facilitates achieving Heisenberg-limited detection at any given numerical aperture. The setup consists of a hollow hemispherical mirror that controls the light scattered by the dipolar emitter, particularly at high scattering angles, thereby focusing the obtained information. This mirror is compatible with existing setups commonly employed in levitated optomechanics, including confocal lenses and optical resonators.

Reports

• R. Blatt MQV K6 Site Visit Erlangen , (2024-06-11), (ID: 721294)
• R. Blatt MQV Site Visit THEQUCO K4, (2024-04-23), (ID: 721291)
• R. Blatt MQV Site Visit TAQC K3, (2024-04-10), (ID: 721289)

2023

Articles

• M. K. Kurmapu, V. V. Tiunova, E. S. Tiunov, M. Ringbauer, C. Maier, R. Blatt, T. Monz, A. K. Fedorov, A. I. Lvovsky Reconstructing Complex States of a 20-Qubit Quantum Simulator, PRX Quantum 4 40345 (2023-12-20), http://dx.doi.org/10.1103/PRXQuantum.4.040345 doi:10.1103/PRXQuantum.4.040345 (ID: 721166) Toggle Abstract
  A prerequisite to the successful development of quantum computers and simulators is precise understanding of the physical processes occurring therein, which can be achieved by measuring the quantum states that they produce. However, the resources required for traditional quantum state estimation scale exponentially with the system size, highlighting the need for alternative approaches. Here, we demonstrate an efficient method for reconstruction of significantly entangled multiqubit quantum states. Using a variational version of the matrix-product-state ansatz, we perform the tomography (in the pure-state approximation) of quantum states produced in a 20-qubit trapped-ion Ising-type quantum simulator, using the data acquired in only 27 bases, with 1000 measurements in each basis. We observe superior state-reconstruction quality and faster convergence compared to the methods based on neural-network quantum state representations: restricted Boltzmann machines and feed-forward neural networks with autoregressive architecture. Our results pave the way toward efficient experimental characterization of complex states produced by the quench dynamics of many-body quantum systems.
• M. K. Joshi, C. Kokail, R. van Bijnen, F. Kranzl, T. Zache, R. Blatt, C. F. Roos, P. Zoller Exploring Large-Scale Entanglement in Quantum Simulation, Nature 624 539 (2023-11-29), http://dx.doi.org/10.1038/s41586-023-06768-0 doi:10.1038/s41586-023-06768-0 (ID: 721080) Toggle Abstract
  Entanglement is a distinguishing feature of quantum many-body systems, and uncovering the entanglement structure for large particle numbers in quantum simulation experiments is a fundamental challenge in quantum information science. Here we perform experimental investigations of entanglement based on the entanglement Hamiltonian, as an effective description of the reduced density operator for large subsystems. We prepare ground and excited states of a 1D XXZ Heisenberg chain on a 51-ion programmable quantum simulator and perform sample-efficient `learning' of the entanglement Hamiltonian for subsystems of up to 20 lattice sites. Our experiments provide compelling evidence for a local structure of the entanglement Hamiltonian. This observation marks the first instance of confirming the fundamental predictions of quantum field theory by Bisognano and Wichmann, adapted to lattice models that represent correlated quantum matter. The reduced state takes the form of a Gibbs ensemble, with a spatially-varying temperature profile as a signature of entanglement. Our results also show the transition from area to volume-law scaling of Von Neumann entanglement entropies from ground to excited states. As we venture towards achieving quantum advantage, we anticipate that our findings and methods have wide-ranging applicability to revealing and understanding entanglement in many-body problems with local interactions including higher spatial dimensions.
• F. Kranzl, S. Birnkammer, M. K. Joshi, A. Bastianello, R. Blatt, M. Knap, C. F. Roos Observation of magnon bound states in the long-range, anisotropic Heisenberg model, Phys. Rev. X 13 031017-12 (2023-08-11), http://dx.doi.org/10.1103/PhysRevX.13.031017 doi:10.1103/PhysRevX.13.031017 (ID: 720909) Toggle Abstract
  Over the recent years coherent, time-periodic modulation has been established as a versatile tool for realizing novel Hamiltonians. Using this approach, known as Floquet engineering, we experimentally realize a long-ranged, anisotropic Heisenberg model with tunable interactions in a trapped ion quantum simulator. We demonstrate that the spectrum of the model contains not only single magnon excitations but also composite magnon bound states. For the long-range interactions with the experimentally realized power-law exponent, the group velocity of magnons is unbounded. Nonetheless, for sufficiently strong interactions we observe bound states of these unconventional magnons which possess a non-diverging group velocity. By measuring the configurational mutual information between two disjoint intervals, we demonstrate the implications of the bound state formation on the entanglement dynamics of the system. Our observations provide key insights into the peculiar role of composite excitations in the non-equilibrium dynamics of quantum many-body systems.
• F. Kranzl, A. Lasek, M. K. Joshi, A. Kalev, R. Blatt, C. F. Roos, N. Yunger Halpern Experimental observation of thermalization with noncommuting charges, PRX Quantum 4 20318 (2023-04-28), http://dx.doi.org/10.1103/PRXQuantum.4.020318 doi:10.1103/PRXQuantum.4.020318 (ID: 720810) Toggle Abstract
  Quantum simulators have recently enabled experimental observations of the internal thermalization of quantum many-body systems. Often, the global energy and particle number are conserved and the system is prepared with a well-defined particle number—in a microcanonical subspace. However, quantum evolution can also conserve quantities, or charges, that fail to commute with each other. Noncommuting charges have recently emerged as a subfield at the intersection of quantum thermodynamics and quantum information. Until now, this subfield has remained theoretical. We initiate the experimental testing of its predictions, with a trapped-ion simulator. We prepare 6–21 spins in an approximate microcanonical subspace, a generalization of the microcanonical subspace for accommodating noncommuting charges, which cannot necessarily have well-defined nontrivial values simultaneously. We simulate a Heisenberg evolution using laser-induced entangling interactions and collective spin rotations. The noncommuting charges are the three spin components. We find that small subsystems equilibrate to near a recently predicted non-Abelian thermal state. This work bridges quantum many-body simulators to the quantum thermodynamics of noncommuting charges, the predictions of which can now be tested.
• P. Hrmo, B. Wilhelm, L. Gerster, M. van Mourik, M. Huber, R. Blatt, P. Schindler, T. Monz, M. Ringbauer Native qudit entanglement in a trapped ion quantum processor, Nat. Commun. 14 doi.org/10.1038/s41467-023-37375-223-37375-2 (2023-04-19), URL (ID: 721076) Toggle Abstract
  Quantum information carriers, just like most physical systems, naturally occupy high-dimensional Hilbert spaces. Instead of restricting them to a two-level subspace, these high-dimensional (qudit) quantum systems are emerging as a powerful resource for the next generation of quantum processors. Yet harnessing the potential of these systems requires efficient ways of generating the desired interaction between them. Here, we experimentally demonstrate an implementation of a native two-qudit entangling gate up to dimension 5 in a trapped-ion system. This is achieved by generalizing a recently proposed light-shift gate mechanism to generate genuine qudit entanglement in a single application of the gate. The gate seamlessly adapts to the local dimension of the system with a calibration overhead that is independent of the dimension.
• J. Franke, S. M. Muleady, C. R. Kaubrügger, F. Kranzl, R. Blatt, A. M. Rey, M. K. Joshi, C. F. Roos Quantum-enhanced sensing on optical transitions through finite-range interactions, Nature 621 740 (2023-03-27), http://dx.doi.org/10.1038/s41586-023-06472-z doi:10.1038/s41586-023-06472-z (ID: 721072) Toggle Abstract
  The control over quantum states in atomic systems has led to the most precise optical atomic clocks to date. Their sensitivity is currently bounded by the standard quantum limit, a fundamental floor set by quantum mechanics for uncorrelated particles, which can nevertheless be overcome when operated with entangled particles. Yet demonstrating a quantum advantage in real world sensors is extremely challenging and remains to be achieved aside from two remarkable examples, LIGO and more recently HAYSTAC. Here we illustrate a pathway for harnessing scalable entanglement in an optical transition using 1D chains of up to 51 ions with state-dependent interactions that decay as a power-law function of the ion separation. We show our sensor can be made to behave as a one-axis-twisting (OAT) model, an iconic fully connected model known to generate scalable squeezing. The collective nature of the state manifests itself in the preservation of the total transverse magnetization, the reduced growth of finite momentum spin-wave excitations, the generation of spin squeezing comparable to OAT (a Wineland parameter of −3.9±0.3 dB for only N = 12 ions) and the development of non-Gaussian states in the form of atomic multi-headed cat states in the Q-distribution. The simplicity of our protocol enables scalability to large arrays with minimal overhead, opening the door to advances in timekeeping as well as new methods for preserving coherence in quantum simulation and computation. We demonstrate this in a Ramsey-type interferometer, where we reduce the measurement uncertainty by −3.2±0.5 dB below the standard quantum limit for N = 51 ions.
• L. O. Conlon, Tobias Vogl, C. Marciniak, Ivan Pogorelov, Simun K. Yung, Falk Eilenberger, D. W. Berry, F. S. Santana, Rainer Blatt, Thomas Monz, Ping Koy Lam, S. M. Assad Approaching optimal entangling collective measurements on quantum computing platforms, Nature Phys. 1-7 (2023-01-12), http://dx.doi.org/10.1038/s41567-022-01875-7 doi:10.1038/s41567-022-01875-7 (ID: 721010) Toggle Abstract
  Entanglement is a fundamental feature of quantum mechanics and holds great promise for enhancing metrology and communications. Much of the focus of quantum metrology so far has been on generating highly entangled quantum states that offer better sensitivity, per resource, than what can be achieved classically. However, to reach the ultimate limits in multi-parameter quantum metrology and quantum information processing tasks, collective measurements, which generate entanglement between multiple copies of the quantum state, are necessary. Here, we experimentally demonstrate theoretically optimal single- and two-copy collective measurements for simultaneously estimating two non-commuting qubit rotations. This allows us to implement quantum-enhanced sensing, for which the metrological gain persists for high levels of decoherence, and to draw fundamental insights about the interpretation of the uncertainty principle. We implement our optimal measurements on superconducting, trapped-ion and photonic systems, providing an indication of how future quantum-enhanced sensing networks may look.

Preprints

• M. I. Hussain, M. Guevara-Bertsch, E. Torrontegui, J. García-Ripoll, R. Blatt, C. F. Roos Single-ion optical autocorrelator, (2023-12-07), arXiv:2312.03679 arXiv:2312.03679 (ID: 721153) Toggle Abstract
  Well isolated quantum systems are exquisite sensors of electromagnetic fields. In this work, we use a single trapped ion for characterizing chirped ultraviolet (UV) picosecond laser pulses. The frequency swept pulses resonantly drive a strong dipole transition via rapid adiabatic passage, resulting in near deterministic population exchange caused by absorption or stimulated emission of photons. When subjecting an ion to counterpropagating pulse pairs, we observe the loss and revival of atomic coherence as a function of the pulse pair spatial overlap enabling quantification of the temporal pulse broadening caused by a frequency chirp in shaped UV pulses with a very low peak power. We find good agreement between measured and applied chirp. The ultrafast population exchange imparts an impulsive force where the estimated change in the mean phonon numbers of 0.5 is measured for two pairs of pulses. The resonant ultrafast kicks could be applied to matter wave interferometry experiments and present a step towards ultrafast entanglement operations in trapped ions.
• M. Meth, J. Haase, J. Zhang, C. Edmunds, L. Postler, A. Steiner, A. Jena, L. Dellantonio, R. Blatt, P. Zoller, T. Monz, P. Schindler, C. A. Muschik, M. Ringbauer Simulating 2D lattice gauge theories on a qudit quantum computer, (2023-10-18), arXiv:2310.12110 arXiv:2310.12110 (ID: 721176) Toggle Abstract
  Particle physics underpins our understanding of the world at a fundamental level by describing the interplay of matter and forces through gauge theories. Yet, despite their unmatched success, the intrinsic quantum mechanical nature of gauge theories makes important problem classes notoriously difficult to address with classical computational techniques. A promising way to overcome these roadblocks is offered by quantum computers, which are based on the same laws that make the classical computations so difficult. Here, we present a quantum computation of the properties of the basic building block of two-dimensional lattice quantum electrodynamics, involving both gauge fields and matter. This computation is made possible by the use of a trapped-ion qudit quantum processor, where quantum information is encoded in d different states per ion, rather than in two states as in qubits. Qudits are ideally suited for describing gauge fields, which are naturally high-dimensional, leading to a dramatic reduction in the quantum register size and circuit complexity. Using a variational quantum eigensolver, we find the ground state of the model and observe the interplay between virtual pair creation and quantized magnetic field effects. The qudit approach further allows us to seamlessly observe the effect of different gauge field truncations by controlling the qudit dimension. Our results open the door for hardware-efficient quantum simulations with qudits in near-term quantum devices.
• M. Ringbauer, M. Hinsche, T. Feldker, P. K. Faehrmann, J. Bermejo-Vega, C. Edmunds, L. Postler, R. Stricker, C. Marciniak, M. Meth, I. Pogorelov, R. Blatt, P. Schindler, J. Eisert, T. Monz, D. Hangleiter Verifiable measurement-based quantum random sampling with trapped ions, (2023-07-26), arXiv:2307.14424 arXiv:2307.14424 (ID: 721154) Toggle Abstract
  Quantum computers are now on the brink of outperforming their classical counterparts. One way to demonstrate the advantage of quantum computation is through quantum random sampling performed on quantum computing devices. However, existing tools for verifying that a quantum device indeed performed the classically intractable sampling task are either impractical or not scalable to the quantum advantage regime. The verification problem thus remains an outstanding challenge. Here, we experimentally demonstrate efficiently verifiable quantum random sampling in the measurement-based model of quantum computation on a trapped-ion quantum processor. We create random cluster states, which are at the heart of measurement-based computing, up to a size of 4 x 4 qubits. Moreover, by exploiting the structure of these states, we are able to recycle qubits during the computation to sample from entangled cluster states that are larger than the qubit register. We then efficiently estimate the fidelity to verify the prepared states--in single instances and on average--and compare our results to cross-entropy benchmarking. Finally, we study the effect of experimental noise on the certificates. Our results and techniques provide a feasible path toward a verified demonstration of a quantum

Report

• R. Blatt 2023 MQV Review Meeting, (2023-09-26), (ID: 721134)

2022

Articles

• R. Stricker, M. Meth, L. Postler, C. Edmunds, Christopher Ferrie, Rainer Blatt, Philipp Schindler, Thomas Monz, R. Kueng, M. Ringbauer Experimental single-setting quantum state tomography, PRX Quantum 3 40310 (2022-12-10), http://dx.doi.org/10.1103/PRXQuantum.3.040310 doi:10.1103/PRXQuantum.3.040310 (ID: 720910)
• R. Stricker, D. Vodola, A. Erhardt, L. Postler, M. Meth, M. Ringbauer, Philipp Schindler, Rainer Blatt, Markus Müller, Thomas Monz Characterizing Quantum Instruments: From Nondemolition Measurements to Quantum Error Correction, PRX Quantum 3 30318 (2022-12-10), http://dx.doi.org/10.1103/PRXQuantum.3.030318 doi:10.1103/PRXQuantum.3.030318 (ID: 720911)
• M. Ringbauer, M. Meth, L. Postler, R. Stricker, Rainer Blatt, Philipp Schindler, Thomas Monz A universal qudit quantum processor with trapped ions, Nature Phys. 18 1053 (2022-12-10), http://dx.doi.org/10.1038/s41567-022-01658-0 doi:10.1038/s41567-022-01658-0 (ID: 720912)
• Lukas Gerster, F. Martinez-Garcia, P. Hrmo, Martin van Mourik, Benjamin Wilhelm, D. Vodola, Markus Müller, Rainer Blatt, Philipp Schindler, Thomas Monz Experimental Bayesian calibration of trapped ion entangling operations, PRX Quantum 3 20350 (2022-12-10), http://dx.doi.org/10.1103/PRXQuantum.3.020350 doi:10.1103/PRXQuantum.3.020350 (ID: 720913)
• L. Postler, S. Heußen, I. Pogorelov, M. Rispler, T. Feldker, M. Meth, C. Marciniak, M. Ringbauer, R. Stricker, Rainer Blatt, Philipp Schindler, Markus Müller, Thomas Monz Demonstration of fault-tolerant universal quantum gate operations, Nature 605 675 (2022-12-10), http://dx.doi.org/10.1038/s41586-022-04721-1 doi:10.1038/s41586-022-04721-1 (ID: 720914)
• M. van Mourik, P. Hrmo, Lukas Gerster, Benjamin Wilhelm, Rainer Blatt, Philipp Schindler, Thomas Monz rf-induced heating dynamics of noncrystallized trapped ions, Phys. Rev. A 105 33101 (2022-12-10), http://dx.doi.org/10.1103/PhysRevA.105.033101 doi:10.1103/PhysRevA.105.033101 (ID: 720915)
• Amit K. Pal, Philipp Schindler, A. Erhardt, A. Rivas, M. A. Martin-Delgado, Rainer Blatt, Thomas Monz, Markus Müller Relaxation times do not capture logical qubit dynamics, Quantum 6 632 (2022-12-10), http://dx.doi.org/10.22331/q-2022-01-24-632 doi:10.22331/q-2022-01-24-632 (ID: 720916)
• F. Kranzl, M. K. Joshi, C. Maier, T. Brydges, J. Franke, R. Blatt, C. F. Roos Controlling long ion strings for quantum simulation and precision measurements, Phys. Rev. A 105 52426 (2022-05-18), http://dx.doi.org/10.1103/PhysRevA.105.052426 doi:10.1103/PhysRevA.105.052426 (ID: 720726) Toggle Abstract
  Scaling a trapped-ion based quantum simulator to a large number of ions creates a fully-controllable quantum system that becomes inaccessible to numerical methods. When highly anisotropic trapping potentials are used to confine the ions in the form of a long linear string, several challenges have to be overcome to achieve high-fidelity coherent control of a quantum system extending over hundreds of micrometers. In this paper, we describe a setup for carrying out many-ion quantum simulations including single-ion coherent control that we use for demonstrating entanglement in 50-ion strings. Furthermore, we present a set of experimental techniques probing ion-qubits by Ramsey and Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences that enable detection (and compensation) of power-line-synchronous magnetic-field variations, measurement of path length fluctuations, and of the wavefronts of elliptical laser beams coupling to the ion string.
• M. K. Joshi, F. Kranzl, A. Schuckert, I. Lovas, C. Maier, R. Blatt, M. Knap, C. F. Roos Observing emergent hydrodynamics in a long-range quantum magnet, Science 376 720 (2022-05-02), http://dx.doi.org/10.1126/science.abk2400 doi:10.1126/science.abk2400 (ID: 720666) Toggle Abstract
  Identifying universal properties of non-equilibrium quantum states is a major challenge in modern physics. A fascinating prediction is that classical hydrodynamics emerges universally in the evolution of any interacting quantum system. Here, we experimentally probe the quantum dynamics of 51 individually controlled ions, realizing a long-range interacting spin chain. By measuring space-time resolved correlation functions in an infinite temperature state, we observe a whole family of hydrodynamic universality classes, ranging from normal diffusion to anomalous superdiffusion, that are described by L\'evy flights. We extract the transport coefficients of the hydrodynamic theory, reflecting the microscopic properties of the system. Our observations demonstrate the potential for engineered quantum systems to provide key insights into universal properties of non-equilibrium states of quantum matter.
• M. Meth, V. Kuzmin, Rick van Bijnen, L. Postler, R. Stricker, Rainer Blatt, M. Ringbauer, Thomas Monz, Pietro Silvi, Philipp Schindler Probing phases of quantum matter with an ion-trap tensor-network quantum eigensolver, Phys. Rev. X 12 41035 (2022-03-28), http://dx.doi.org/10.1103/PhysRevX.12.041035 doi:10.1103/PhysRevX.12.041035 (ID: 720826) Toggle Abstract
  Tensor-Network (TN) states are efficient parametric representations of ground states of local quantum Hamiltonians extensively used in numerical simulations. Here we encode a TN ansatz state directly into a quantum simulator, which can potentially offer an exponential advantage over purely numerical simulation. In particular, we demonstrate the optimization of a quantum-encoded TN ansatz state using a variational quantum eigensolver on an ion-trap quantum computer by preparing the ground states of the extended Su-Schrieffer-Heeger model. The generated states are characterized by estimating the topological invariants, verifying their topological order. Our TN encoding as a trapped ion circuit employs only single-site addressing optical pulses - the native operations naturally available on the platform. We reduce nearest-neighbor crosstalk by selecting different magnetic sublevels with well-separated transition frequencies to encode even and odd qubits.
• C. Marciniak, T. Feldker, I. Pogorelov, C. R. Kaubrügger, D. Vasilyev, R. van Bijnen, P. Schindler, P. Zoller, R. Blatt, T. Monz Optimal metrology with variational quantum circuits on trapped ions, Nature 603 604 (2022-03-23), http://dx.doi.org/10.1038/s41586-022-04435-4 doi:10.1038/s41586-022-04435-4 (ID: 720667) Toggle Abstract
  Cold atoms and ions exhibit unparalleled performance in frequency metrology epitomized in the atomic clock. More recently, such atomic systems have been used to implement programmable quantum computers and simulators with highest reported operational fidelities across platforms. Their strength in metrology and quantum information processing offers the opportunity to utilize tailored, programmable entanglement generation to approach the `optimal quantum sensor' compatible with quantum mechanics. Here we report quantum enhancement in metrology beyond squeezing through low-depth, variational quantum circuits searching for optimal input states and measurement operators in a trapped ion platform. We perform entanglement-enhanced Ramsey interferometry finding optimal parameters for variational quantum circuits using a Bayesian approach to stochastic phase estimation tailored to the sensor platform capabilities and finite dynamic range of the interferometer. We verify the performance by both directly using theory predictions of optimal parameters, and performing online quantum-classical feedback optimization to `self-calibrate' the variational parameters. In both cases we find that variational circuits outperform classical and direct spin squeezing strategies under realistic noise and imperfections. With 26 ions we achieve 2.02(8) dB of metrological gain over classical interferometers.
• S. Auchter , C. Axline, C. Decaroli, M. Valentini, L. Purwin, R. Oswald, R. Matt, Elmar Aschauer, Y. Colombe, Philip Holz, Thomas Monz, Rainer Blatt, Philipp Schindler, C. Rössler, J. Home Industrially Microfabricated Ion Trap with 1 eV Trap Depth, Quantum Sci. Technol. 7 35015 (2022-02-17), http://dx.doi.org/10.1088/2058-9565/ac7072 doi:10.1088/2058-9565/ac7072 (ID: 720813) Toggle Abstract
  Scaling trapped-ion quantum computing will require robust trapping of at least hundreds of ions over long periods, while increasing the complexity and functionality of the trap itself. Symmetric 3D structures enable high trap depth, but microfabrication techniques are generally better suited to planar structures that produce less ideal conditions for trapping. We present an ion trap fabricated on stacked 8-inch wafers in a large-scale MEMS microfabrication process that provides reproducible traps at a large volume. Electrodes are patterned on the surfaces of two opposing wafers bonded to a spacer, forming a 3D structure with 2.5 micrometer standard deviation in alignment across the stack. We implement a design achieving a trap depth of 1 eV for a calcium-40 ion held at 200 micrometers from either electrode plane. We characterize traps, achieving measurement agreement with simulations to within +/-5% for mode frequencies spanning 0.6--3.8 MHz, and evaluate stray electric field across multiple trapping sites. We measure motional heating rates over an extensive range of trap frequencies, and temperatures, observing 40 phonons/s at 1 MHz and 185 K. This fabrication method provides a highly scalable approach for producing a new generation of 3D ion traps.

Reports

• Rainer Blatt MQV Scientific Report 2022 , Mitgliederversammlung von Munich Quantum Valley e.V. , (2022-07-13), (ID: 720871)
• Rainer Blatt Third Meeting of the Dioscuri Committee in the 4th Call for DCs in PL, (2022-01-19), (ID: 720852)

2021

Articles

• A. Seshadri, M. Ringbauer, R. Blatt, T. Monz, S. Becker Versatile fidelity estimation with confidence, PRL 133 020402-1 (2021-12-15), http://dx.doi.org/10.1103/PhysRevLett.133.020402 doi:10.1103/PhysRevLett.133.020402 (ID: 720738) Toggle Abstract
  As quantum devices become more complex and the requirements on these devices become more demanding, it is crucial to be able to verify the performance of such devices in a scalable and reliable fashion. A cornerstone task in this challenge is quantifying how close an experimentally prepared quantum state is to the desired one. Here we present a method to construct an estimator for the quantum state fidelity that is compatible with any measurement protocol. Our method provides a confidence interval on this estimator that is guaranteed to be nearly minimax optimal for the specified measurement protocol. For a well-chosen measurement scheme, our method is competitive in the number of measurement outcomes required for estimation. We demonstrate our method using simulations and experimental data from a trapped-ion quantum computer and compare the results to state-of-the-art techniques. Our method can be easily extended to estimate the expectation value of any observable, such as entanglement witnesses.
• G. cerchiari, L. Dania, D. Bykov, R. Blatt, T. E. Northup Position measurement of a dipolar scatterer via self-homodyne detection, Phys. Rev. A 104 53523 (2021-11-19), http://dx.doi.org/10.1103/PhysRevA.104.053523 doi:10.1103/PhysRevA.104.053523 (ID: 720734) Toggle Abstract
  We describe a technique to measure the position of a dipolar scatterer based on self-homodyne detection of the scattered light. The method can theoretically reach the Heisenberg limit, at which information gained about the position is constrained only by the back action of the scattered light. The technique has applications in the fields of levitated optomechanics and trapped ions and is generally applicable to the position determination of confined light scatterers.
• C. Greganti, T. F. Demarie, M. Ringbauer, J. A. Jones, V. Saggio, I. A. Calafell, L. A. Rozema, A. Erhard, M. Meth, L. Postler, R. Stricker, P. Schindler, R. Blatt, T. Monz, P. Walther, J. F. Fitzsimons Verification of independent quantum devices, Phys. Rev. X 11 31049 (2021-09-01), http://dx.doi.org/10.1103/PhysRevX.11.031049 doi:10.1103/PhysRevX.11.031049 (ID: 720502) Toggle Abstract
  Quantum computers are on the brink of surpassing the capabilities of even the most powerful classical computers. This naturally raises the question of how one can trust the results of a quantum computer when they cannot be compared to classical simulation. Here we present a verification technique that exploits the principles of measurement-based quantum computation to link quantum circuits of different input size, depth, and structure. Our approach enables consistency checks of quantum computations within a device, as well as between independent devices. We showcase our protocol by applying it to five state-of-the-art quantum processors, based on four distinct physical architectures: nuclear magnetic resonance, superconducting circuits, trapped ions, and photonics, with up to 6 qubits and 200 distinct circuits.
• G. cerchiari, G. Araneda, L. Podhora, L. Slodicka, Y. Colombe, R. Blatt Measuring ion oscillations at the quantum level with fluorescence light, Phys. Rev. Lett. 127 63603 (2021-08-04), http://dx.doi.org/10.1103/PhysRevLett.127.063603 doi:10.1103/PhysRevLett.127.063603 (ID: 720535) Toggle Abstract
  We demonstrate an optical method for detecting the mechanical oscillations of an atom with single-phonon sensitivity. The measurement signal results from the interference between the light scattered by a single trapped atomic ion and that of its mirror image. The motion of the atom modulates the interference path length and hence the photon detection rate. We detect the oscillations of the atom in the Doppler cooling limit and reconstruct average trajectories in phase space. We demonstrate single-phonon sensitivity near the ground state of motion after EIT cooling. These results could be applied for motion detection of other light scatterers of fundamental interest, such as trapped nanoparticles.
• I. Pogorelov, T. Feldker, C. Marciniak, G. Jacob, V. Podlesnic, M. Meth, V. Negnevitsky, M. Stadler, K. Lakhmanskiy, R. Blatt, P. Schindler, T. Monz A compact ion-trap quantum computing demonstrator, PRX Quantum 2 20342 (2021-06-17), http://dx.doi.org/10.1103/PRXQuantum.2.020343 doi:10.1103/PRXQuantum.2.020343 (ID: 720642) Toggle Abstract
  Quantum information processing is steadily progressing from a purely academic discipline towards applications throughout science and industry. Transitioning from lab-based, proof-of-concept experiments to robust, integrated realizations of quantum information processing hardware is an important step in this process. However, the nature of traditional laboratory setups does not offer itself readily to scaling up system sizes or allow for applications outside of laboratory-grade environments. This transition requires overcoming challenges in engineering and integration without sacrificing the state-of-the-art performance of laboratory implementations. Here, we present a 19-inch rack quantum computing demonstrator based on 40Ca+ optical qubits in a linear Paul trap to address many of these challenges. We outline the mechanical, optical, and electrical subsystems. Further, we describe the automation and remote access components of the quantum computing stack. We conclude by describing characterization measurements relevant to digital quantum computing including entangling operations mediated by the Molmer-Sorenson interaction. Using this setup we produce maximally-entangled Greenberger-Horne-Zeilinger states with up to 24 ions without the use of post-selection or error mitigation techniques; on par with well-established conventional laboratory setups.
• M. Teller, D. Fioretto, P. Holz, P. Schindler, V. Messerer, K. Schüppert, Y. Zou, R. Blatt, J. Chiaverini, J. Sage, T. E. Northup Heating of a Trapped Ion Induced by Dielectric Materials, 126 230505 (2021-06-14), http://dx.doi.org/10.1103/PhysRevLett.126.230505 doi:10.1103/PhysRevLett.126.230505 (ID: 720661) Toggle Abstract
  Electric-field noise due to surfaces disturbs the motion of nearby trapped ions, compromising the fidelity of gate operations that are the basis for quantum computing algorithms. We present a method that predicts the effect of dielectric materials on the ion’s motion. Such dielectrics are integral components of ion traps. Quantitative agreement is found between a model with no free parameters and measurements of a trapped ion in proximity to dielectric mirrors. We expect that this approach can be used to optimize the design of ion-trap-based quantum computers and network nodes.
• P. Holz, K. Lakhmanskiy, D. Rathje, P. Schindler, Y. Colombe, R. Blatt Electric field noise in a high-temperature superconducting surface ion trap, Phys. Rev. B 104 64513 (2021-06-09), http://dx.doi.org/10.1103/PhysRevB.104.064513 doi:10.1103/PhysRevB.104.064513 (ID: 720658) Toggle Abstract
  Scaling up trapped-ion quantum computers requires new trap materials to be explored. Here, we present experiments with a surface ion trap made from the high-temperature superconductor YBCO, a promising material for future trap designs. We show that voltage noise from superconducting electrode leads is negligible within the sensitivity SV=9×10−20V2Hz−1 of our setup, and for lead dimensions typical for advanced trap designs. Furthermore, we investigate the frequency and temperature dependence of electric field noise above a YBCO surface. We find a 1/f spectral dependence of the noise and a non-trivial temperature dependence, with a plateau in the noise stretching over roughly 60K. The onset of the plateau coincides with the superconducting transition, indicating a connection between the dominant noise and the YBCO trap material. We exclude the YBCO bulk as origin of the noise and suggest further experiments to decide between the two remaining options explaining the observed temperature dependence: noise screening within the superconducting phase, or surface noise activated by the YBCO bulk through some unknown mechanism.
• G. cerchiari, G. Araneda, L. Podhora, L. Slodicka, Y. Colombe, R. Blatt Motion analysis of a trapped ion chain by single photon self-interference, Appl. Phys. Lett. 119 24003 (2021-04-16), http://dx.doi.org/10.1063/5.0052099 doi:10.1063/5.0052099 (ID: 720641) Toggle Abstract
  We present an optical scheme to detect the oscillations of a two-ion string confined in a linear Paul trap. The motion is detected by analyzing the intensity correlations in the fluorescence light emitted by one or two ions in the string. We present measurements performed under continuous Doppler cooling and under pulsed illumination. We foresee several direct applications of this detection method, including motional analysis of multi-ion species or coupled mechanical oscillators, and sensing of mechanical correlations.
• M. I. Hussain, D. Heinrich, M. Guevara-Bertsch, E. Torrontegui, J. J. Garcia-Ripoll, C. F. Roos, R. Blatt Ultraviolet Laser Pulses with Multigigahertz Repetition Rate and Multiwatt Average Power for Fast Trapped-Ion Entanglement Operations, Phys. Rev. Applied 15 24054 (2021-02-23), http://dx.doi.org/10.1103/PhysRevApplied.15.024054 doi:10.1103/PhysRevApplied.15.024054 (ID: 720517) Toggle Abstract
  The conventional approach to perform two-qubit gate operations in trapped ions relies on exciting the ions on motional sidebands with laser light, which is an inherently slow process. One way to implement a fast entangling gate protocol requires a suitable pulsed laser to increase the gate speed by orders of magnitude. However, the realization of such a fast entangling gate operation presents a big technical challenge, as such the required laser source is not available off-the-shelf. For this, we have engineered an ultrafast entangling gate source based on a frequency comb. The laser generates bursts of several hundred mode-locked pulses with pulse energy ∼800 pJ at 5 GHz repetition rate at 393.3 nm and complies with all requirements for implementing a fast two-qubit gate operation. To verify the applicability and projected performance we run simulations based on our source parameters. The gate time can be faster than a trap period with an error approaching 10^(−4).
• A. Erhard, H. Poulsen Nautrup, M. Meth, L. Postler, R. Stricker, M. Ringbauer, P. Schindler, H. J. Briegel, R. Blatt, N. Friis, T. Monz Entangling logical qubits with lattice surgery, Nature 589 224 (2021-01-13), http://dx.doi.org/10.1038/s41586-020-03079-6 doi:10.1038/s41586-020-03079-6 (ID: 720515) Toggle Abstract
  Future quantum computers will require quantum error correction for faithful operation. The correction capabilities come with an overhead for performing fault-tolerant logical operations on the encoded qubits. One of the most resource efficient ways to implement logical operations is lattice surgery, where groups of physical qubits, arranged on lattices, can be merged and split to realize entangling gates and teleport logical information. Here, we report on the experimental realization of lattice surgery between two topologically encoded qubits in a 10-qubit ion trap quantum information processor. In particular, we demonstrate entanglement between two logical qubits and we implement logical state teleportation.

Preprints

• L. Gerster, F. Martínez-García, P. Hrmo, M. van Mourik, B. Wilhelm, D. Vodola, M. Mueller, R. Blatt, P. Schindler, T. Monz Experimental Bayesian calibration of trapped ion entangling operations, (2021-12-02), arXiv:2112.01411 arXiv:2112.01411 (ID: 720735) Toggle Abstract
  The performance of quantum gate operations is experimentally determined by how correct operational parameters can be determined and set, and how stable these parameters can be maintained. In addition, gates acting on different sets of qubits require unique sets of control parameters. Thus, an efficient multi-dimensional parameter estimation procedure is crucial to calibrate even medium sized quantum processors. Here, we develop and characterize an efficient calibration protocol to automatically estimate and adjust experimental parameters of the widely used Molmer-Sorensen entangling gate operation in a trapped ion quantum information processor. The protocol exploits Bayesian parameter estimation methods which includes a stopping criterion based on a desired gate infidelity. We experimentally demonstrate a median gate infidelity of 1.3(1)⋅10−3, requiring only 1200±500 experimental cycles, while completing the entire gate calibration procedure in less than one minute. This approach is applicable to other quantum information processor architectures with known or sufficiently characterized theoretical models.
• L. Postler, S. Heußen, I. Pogorelov, M. Rispler, T. Feldker, M. Meth, C. Marciniak, R. Stricker, M. Ringbauer, R. Blatt, P. Schindler, M. Mueller, T. Monz Demonstration of fault-tolerant universal quantum gate operations, (2021-11-24), arXiv:2111.12654 arXiv:2111.12654 (ID: 720736) Toggle Abstract
  Quantum computers can be protected from noise by encoding the logical quantum information redundantly into multiple qubits using error correcting codes. When manipulating the logical quantum states, it is imperative that errors caused by imperfect operations do not spread uncontrollably through the quantum register. This requires that all operations on the quantum register obey a fault-tolerant circuit design which, in general, increases the complexity of the implementation. Here, we demonstrate a fault-tolerant universal set of gates on two logical qubits in a trapped-ion quantum computer. In particular, we make use of the recently introduced paradigm of flag fault tolerance, where the absence or presence of dangerous errors is heralded by usage of few ancillary 'flag' qubits. We perform a logical two-qubit CNOT-gate between two instances of the seven qubit color code, and we also fault-tolerantly prepare a logical magic state. We then realize a fault-tolerant logical T-gate by injecting the magic state via teleportation from one logical qubit onto the other. We observe the hallmark feature of fault tolerance, a superior performance compared to a non-fault-tolerant implementation. In combination with recently demonstrated repeated quantum error correction cycles these results open the door to error-corrected universal quantum computation.
• M. Ringbauer, M. Meth, L. Postler, R. Stricker, R. Blatt, P. Schindler, T. Monz A universal qudit quantum processor with trapped ions, (2021-09-14), arXiv:2109.06903 arXiv:2109.06903 (ID: 720737) Toggle Abstract
  Today's quantum computers operate with a binary encoding that is the quantum analog of classical bits. Yet, the underlying quantum hardware consists of information carriers that are not necessarily binary, but typically exhibit a rich multilevel structure, which is artificially restricted to two dimensions. A wide range of applications from quantum chemistry to quantum simulation, on the other hand, would benefit from access to higher-dimensional Hilbert spaces, which conventional quantum computers can only emulate. Here we demonstrate a universal qudit quantum processor using trapped ions with a local Hilbert space dimension of up to 7. With a performance similar to qubit quantum processors, this approach enables native simulation of high-dimensional quantum systems, as well as more efficient implementation of qubit-based algorithms.
• M. van Mourik, P. Hrmo, L. Gerster, B. Wilhelm, R. Blatt, P. Schindler, T. Monz RF-induced heating dynamics of non-crystallized trapped ions , (2021-04-22), arXiv:2104.10623 arXiv:2104.10623 (ID: 720645) Toggle Abstract
  We investigate the energy dynamics of non-crystallized (melted) ions, confined in a Paul trap. The non-periodic Coulomb interaction experienced by melted ions forms a medium for non-conservative energy transfer from the radio-frequency (rf) field to the ions, a process known as rf heating. We study rf heating by analyzing numerical simulations of non-crystallized ion motion in Paul trap potentials, in which the energy of the ions' secular motion changes at discrete intervals, corresponding to ion-ion collisions. The analysis of these collisions is used as a basis to derive a simplified model of rf heating energy dynamics, from which we conclude that the rf heating rate is predominantly dependent on the rf field strength. We confirm the predictability of the model experimentally: Two trapped 40Ca+ ions are deterministically driven to melt, and their fluorescence rate is used to infer the ions' energy. From simulation and experimental results, we generalize which experimental parameters are required for efficient recrystallization of melted trapped ions.

2020

Articles

• M. C. Bañuls, R. Blatt, J. Catani, A. Celi, J. I. Cirac, M. Dalmonte, L. Fallani, K. Jansen, M. Lewenstein, S. Montangero, C. A. Muschik, B. Reznik, E. Rico Ortega, L. Tagliacozzo, K. Van Acoleyen, F. Verstraete, U. Wiese, M. Wingate, J. Zakrzewski, P. Zo Simulating Lattice Gauge Theories within Quantum Technologies, The European Physical Journal D 74 165 (2020-08-04), http://dx.doi.org/10.1140/epjd/e2020-100571-8 doi:10.1140/epjd/e2020-100571-8 (ID: 720395) Toggle Abstract
  Lattice gauge theories, which originated from particle physics in the context of Quantum Chromodynamics (QCD), provide an important intellectual stimulus to further develop quantum information technologies. While one long-term goal is the reliable quantum simulation of currently intractable aspects of QCD itself, lattice gauge theories also play an important role in condensed matter physics and in quantum information science. In this way, lattice gauge theories provide both motivation and a framework for interdisciplinary research towards the development of special purpose digital and analog quantum simulators, and ultimately of scalable universal quantum computers. In this manuscript, recent results and new tools from a quantum science approach to study lattice gauge theories are reviewed. Two new complementary approaches are discussed: first, tensor network methods are presented - a classical simulation approach - applied to the study of lattice gauge theories together with some results on Abelian and non-Abelian lattice gauge theories. Then, recent proposals for the implementation of lattice gauge theory quantum simulators in different quantum hardware are reported, e.g., trapped ions, Rydberg atoms, and superconducting circuits. Finally, the first proof-of-principle trapped ions experimental quantum simulations of the Schwinger model are reviewed.
• E. Torrontegui, D. Heinrich, M. I. Hussain, R. Blatt, J. J. Garcia-Ripoll Ultra-fast two-qubit ion gate using sequences of resonant pulses, New J. Phys. 22 103024 (2020-07-03), 103024 103024 (ID: 720513) Toggle Abstract
  We propose a new protocol to implement ultra-fast two-qubit phase gates with trapped ions using spin-dependent kicks induced by resonant transitions. By only optimizing the allocation of the arrival times in a pulse train sequence the gate is implemented in times faster than the trapping oscillation period T<2*pi/ω. Such gates allow us to increase the number of gate operations that can be completed within the coherence time of the ion-qubits favoring the development of scalable quantum computers.
• M. K. Joshi, A. Fabre, C. Maier, T. Brydges, D. Kiesenhofer, H. Hainzer, R. Blatt, C. F. Roos Polarization-gradient cooling of 1D and 2D ion Coulomb crystals, New J. Phys. 22 103013 (2020-06-29), http://dx.doi.org/10.1088/1367-2630/abb912 doi:10.1088/1367-2630/abb912 (ID: 720512) Toggle Abstract
  We present experiments on polarization gradient cooling of Ca+ multi-ion Coulomb crystals in a linear Paul trap. Polarization gradient cooling of the collective modes of motion whose eigenvectors have overlap with the symmetry axis of the trap is achieved by two counter-propagating laser beams with mutually orthogonal linear polarizations that are blue-detuned from the S1/2 to P1/2 transition. We demonstrate cooling of linear chains of up to 51 ions and 2D-crystals in zig-zag configuration with 22 ions. The cooling results are compared with numerical simulations and the predictions of a simple model of cooling in a moving polarization gradient.
• P. Holz, S. Auchter , G. Stocker, M. Valentini, K. Lakhmanskiy, P. Stampfer, S. Sgouridis, E. Aschauer, Y. Colombe, R. Blatt, C. Rössler Two-dimensional linear trap array for quantum information processing, Advanced Quantum Technologies 3 2000031 (2020-05-18), http://dx.doi.org/10.1002/qute.202000031 doi:10.1002/qute.202000031 (ID: 720490)
• A. Borne, T. E. Northup, R. Blatt, B. Dayan Efficient ion-photon qubit SWAP gate in realistic ion cavity-QED systems without strong coupling, Opt. Express 28 11839 (2020-04-13), http://dx.doi.org/10.1364/OE.376914 doi:10.1364/OE.376914 (ID: 720492) Toggle Abstract
  We present a scheme for deterministic ion-photon qubit exchange, namely a SWAP gate, based on realistic cavity-QED systems with 171Yb+, 40Ca+ and 138Ba+ ions. The gate can also serve as a single-photon quantum memory, in which an outgoing photon heralds the successful arrival of the incoming photonic qubit. Although strong coupling, namely having the single-photon Rabi frequency be the fastest rate in the system, is often assumed essential, this gate (similarly to the Duan-Kimble C-phase gate) requires only Purcell enhancement, i.e. high single-atom cooperativity. Accordingly, it does not require small mode volume cavities, which are challenging to incorporate with ions due to the difficulty of trapping them close to dielectric surfaces. Instead, larger cavities, potentially more compatible with the trap apparatus, are sufficient, as long as their numerical aperture is high enough to maintain small mode area at the ion’s position. We define the optimal parameters for the gate’s operation and simulate the expected fidelities and efficiencies, demonstrating that efficient photon-ion qubit exchange, a valuable building block for scalable quantum computation, is practically attainable with current experimental capabilities.
• F. Ong, K. Schüppert, P. Jobez, M. Teller, B. Ames, D. Fioretto, K. Friebe, M. Lee, Y. Colombe, R. Blatt, T. E. Northup Probing surface charge densities on optical fibers with a trapped ion, New J. Phys. 22 63018 (2020-03-30), http://dx.doi.org/10.1088/1367-2630/ab8af9 doi:10.1088/1367-2630/ab8af9 (ID: 720489)
• R. Stricker, D. Vodola, A. Erhard, L. Postler, M. Meth, M. Ringbauer, P. Schindler, T. Monz, M. Müller, R. Blatt Experimental deterministic correction of qubit loss, Nature 585 207 (2020-02-21), http://dx.doi.org/10.1038/s41586-020-2667-0 doi:10.1038/s41586-020-2667-0 (ID: 720491)
• M. van Mourik, E. A. Martínez, L. Gerster, P. Hrmo, T. Monz, P. Schindler, R. Blatt Coherent rotations of qubits within a multi-species ion-trap quantum computer, Phys. Rev. A 102 22611 (2020-01-08), http://dx.doi.org/10.1103/PhysRevA.102.022611 doi:10.1103/PhysRevA.102.022611 (ID: 720488)
• M. K. Joshi, A. Elben, B. Vermersch, T. Brydges, C. Maier, P. Zoller, R. Blatt, C. F. Roos Quantum information scrambling in a trapped-ion quantum simulator with tunable range interactions, Phys. Rev. Lett. 124 240505 (2020-01-07), http://dx.doi.org/10.1103/PhysRevLett.124.240505 doi:10.1103/PhysRevLett.124.240505 (ID: 720436) Toggle Abstract
  In ergodic many-body quantum systems, locally encoded quantum information becomes, in the course of time evolution, inaccessible to local measurements. This concept of "scrambling" is currently of intense research interest, entailing a deep understanding of many-body dynamics such as the processes of chaos and thermalization. Here, we present first experimental demonstrations of quantum information scrambling on a 10-qubit trapped-ion quantum simulator representing a tunable long-range interacting spin system, by estimating out-of-time ordered correlators (OTOCs) through randomized measurements. We also analyze the role of decoherence in our system by comparing our measurements to numerical simulations and by measuring Rényi entanglement entropies.
• A. Elben, B. Vermersch, R. van Bijnen, C. Kokail, T. Brydges, C. Maier, M. K. Joshi, R. Blatt, C. F. Roos, P. Zoller Cross-Platform Verification of Intermediate Scale Quantum Devices, Phys. Rev. Lett. 124 10504 (2020-01-06), http://dx.doi.org/10.1103/PhysRevLett.124.010504 doi:10.1103/PhysRevLett.124.010504 (ID: 720357) Toggle Abstract
  We describe a protocol for cross-platform verification of quantum simulators and quantum computers. We show how to measure directly the overlap Tr[ρ1ρ2] and the purities Tr[ρ21,2], and thus a (mixed-state) fidelity, of two quantum states ρ1 and ρ2 prepared in separate experimental platforms. We require only local measurements in randomized product bases, which are communicated classically. As a proof-of-principle, we present the measurement of experiment-theory fidelities for entangled 10-qubit quantum states in a trapped ion quantum simulator.

Preprint

• A. K. Pal, P. Schindler, A. Erhardt, A. Rivas, M. A. Martin-Delgado, R. Blatt, T. Monz, M. Müller Relaxation times do not capture logical qubit dynamics, (2020-12-01), arXiv:2012.07911 arXiv:2012.07911 (ID: 720759) Toggle Abstract
  Quantum error correction procedures have the potential to enable faithful operation of large-scale quantum computers. They protect information from environmental decoherence by storing it in logical qubits, built from ensembles of entangled physical qubits according to suitably tailored quantum error correcting encodings. To date, no generally accepted framework to characterise the behaviour of logical qubits as quantum memories has been developed. In this work, we show that generalisations of well-established figures of merit of physical qubits, such as relaxation times, to logical qubits fail and do not capture dynamics of logical qubits. We experimentally illustrate that, in particular, spatial noise correlations can give rise to rich and counter-intuitive dynamical behavior of logical qubits. We show that a suitable set of observables, formed by code space population and logical operators within the code space, allows one to track and characterize the dynamical behaviour of logical qubits. Awareness of these effects and the efficient characterisation tools used in this work will help to guide and benchmark experimental implementations of logical qubits.

2019

Articles

• A. Erhard, J. J. Wallman, L. Postler, M. Meth, R. Stricker, E. A. Martínez, P. Schindler, T. Monz, J. Emerson, R. Blatt Characterizing large-scale quantum computers via cycle benchmarking, Nat. Commun. 10 5347 (2019-11-25), (ID: 720497) Toggle Abstract
  Quantum computers promise to solve certain problems more efficiently than their digital counterparts. A major challenge towards practically useful quantum computing is characterizing and reducing the various errors that accumulate during an algorithm running on large-scale processors. Current characterization techniques are unable to adequately account for the exponentially large set of potential errors, including cross-talk and other correlated noise sources. Here we develop cycle benchmarking, a rigorous and practically scalable protocol for characterizing local and global errors across multi-qubit quantum processors. We experimentally demonstrate its practicality by quantifying such errors in non-entangling and entangling operations on an ion-trap quantum computer with up to 10 qubits, and total process fidelities for multi-qubit entangling gates ranging from 99.6(1)% for 2 qubits to 86(2)% for 10 qubits. Furthermore, cycle benchmarking data validates that the error rate per single-qubit gate and per two-qubit coupling does not increase with increasing system size.
• Moonjoo Lee, Minjae Lee, S. Hong, K. Schüppert, Y. D. Kwon, T. Kim, Y. Colombe, T. E. Northup, D. D. Cho, R. Blatt MEMS-based design of a high-finesse fiber cavity integrated with an ion trap, Phys. Rev. Applied 12 (2019-10-23), http://dx.doi.org/10.1103/PhysRevApplied.12.044052 doi:10.1103/PhysRevApplied.12.044052 (ID: 720498) Toggle Abstract
  We present a numerical study of a microelectromechanical-system-based design of a fiber cavity integrated with an ion-trap system. Each fiber mirror is supported by a microactuator that controls the mirror’s position in three dimensions. The mechanical stability is investigated by a feasibility analysis, which shows that the actuator offers stable support of the fiber. The actuators move the fibers’ positions continuously with a stroke of more than 10μm, with mechanical resonance frequencies on the order of kilohertz. A calculation of the trapping potential shows that a separation between the ion and the fiber consistent with strong ion-cavity coupling is feasible. Our miniaturized ion-photon interface constitutes a viable approach to integrated hardware for quantum information.
• G. Araneda, R. Walser, Y. Colombe, D. Higginbottom, J. Volz, R. Blatt, A. Rauschenbeutel "Wavelength-scale errors in optical localization due to spin–orbit coupling of light, Nature Phys. 15 21 (2019-08-20), http://dx.doi.org/10.1038/s41567-018-0301-y doi:10.1038/s41567-018-0301-y (ID: 720501) Toggle Abstract
  The precise determination of the position of point-like emitters and scatterers using far-field optical imaging techniques is of utmost importance for a wide range of applications in medicine, biology, astronomy, and physics. Although the optical wavelength sets a fundamental limit to the image resolution of unknown objects, the position of an individual emitter can in principle be estimated from the image with arbitrary precision. This is used, e.g., in stars' position determination and in optical super-resolution microscopy. Furthermore, precise position determination is an experimental prerequisite for the manipulation and measurement of individual quantum systems, such as atoms, ions, and solid state-based quantum emitters. Here we demonstrate that spin-orbit coupling of light in the emission of elliptically polarized emitters can lead to systematic, wavelength-scale errors in the estimate of the emitter's position. Imaging a single trapped atom as well as a single sub-wavelength-diameter gold nanoparticle, we demonstrate a shift between the emitters' measured and actual positions which is comparable to the optical wavelength. Remarkably, for certain settings, the expected shift can become arbitrarily large. Beyond their relevance for optical imaging techniques, our findings apply to the localization of objects using any type of wave that carries orbital angular momentum relative to the emitter's position with a component orthogonal to the direction of observation.
• D. Heinrich, M. Guggemos, M. Guevara-Bertsch, M. I. Hussain, C. F. Roos, R. Blatt Ultrafast coherent excitation of a Ca+ ion, New J. Phys. 21 83025 (2019-07-01), http://dx.doi.org/10.1088/1367-2630/ab2a7e doi:10.1088/1367-2630/ab2a7e (ID: 720108) Toggle Abstract
  Trapped ions are a well-studied and promising system for the realization of a scalable quantum computer. Faster quantum gates would greatly improve the applicability of such a system and allow for greater flexibility in the number of calculation steps. In this paper we present a pulsed laser system, delivering picosecond pulses at a repetition rate of 5 GHz and resonant to the S1/2 to P3/2 transition in Ca+ for coherent population transfer to implement fast phase gate operations. The optical pulse train is derived from a mode-locked, stabilized optical frequency comb and inherits its frequency stability. Using a single trapped ion, we implement three different techniques for measuring the ion-laser coupling strength and characterizing the pulse train emitted by the laser, and show how all requirements can be met for an implementation of a fast phase gate operation.
• C. Kokail, C. Maier, R. van Bijnen, T. Brydges, M. K. Joshi, P. Jurcevic, C. A. Muschik, P. Silvi, R. Blatt, C. F. Roos, P. Zoller Self-verifying variational quantum simulation of lattice models, Nature 569 360 (2019-05-15), http://dx.doi.org/10.1038/s41586-019-1177-4 doi:10.1038/s41586-019-1177-4 (ID: 720076) Toggle Abstract
  Hybrid classical-quantum algorithms aim at variationally solving optimisation problems, using a feedback loop between a classical computer and a quantum co-processor, while benefitting from quantum resources. Here we present experiments demonstrating self-verifying, hybrid, variational quantum simulation of lattice models in condensed matter and high-energy physics. Contrary to analog quantum simulation, this approach forgoes the requirement of realising the targeted Hamiltonian directly in the laboratory, thus allowing the study of a wide variety of previously intractable target models. Here, we focus on the Lattice Schwinger model, a gauge theory of 1D quantum electrodynamics. Our quantum co-processor is a programmable, trapped-ion analog quantum simulator with up to 20 qubits, capable of generating families of entangled trial states respecting symmetries of the target Hamiltonian. We determine ground states, energy gaps and, by measuring variances of the Schwinger Hamiltonian, we provide algorithmic error bars for energies, thus addressing the long-standing challenge of verifying quantum simulation.
• M. Mamaev, R. Blatt, J. Ye, A. M. Rey Cluster State Generation with Spin-Orbit Coupled Fermionic Atoms in Optical Lattices, Phys. Rev. Lett. 122 160402 (2019-04-27), http://dx.doi.org/10.1103/PhysRevLett.122.160402 doi:10.1103/PhysRevLett.122.160402 (ID: 720107) Toggle Abstract
  Measurement-based quantum computation, an alternative paradigm for quantum information processing, uses simple measurements on qubits prepared in cluster states, a class of multiparty entangled states with useful properties. Here we propose and analyze a scheme that takes advantage of the interplay between spin-orbit coupling and superexchange interactions, in the presence of a coherent drive, to deterministically generate macroscopic arrays of cluster states in fermionic alkaline earth atoms trapped in three dimensional (3D) optical lattices. The scheme dynamically generates cluster states without the need of engineered transport, and is robust in the presence of holes, a typical imperfection in cold atom Mott insulators. The protocol is of particular relevance for the new generation of 3D optical lattice clocks with coherence times >10 s, two orders of magnitude larger than the cluster state generation time. We propose the use of collective measurements and time-reversal of the Hamiltonian to benchmark the underlying Ising model dynamics and the generated many-body correlations.
• M. Guggemos, M. Guevara-Bertsch, D. Heinrich, Ó. A. Herrera, Y. Colombe, R. Blatt, C. F. Roos Frequency measurement of the 1S0,F=5/2 to 3P1,F=7/2 transition of 27Al+ via quantum logic spectroscopy with 40Ca+, New J. Phys. 21 103003 (2019-04-25), http://dx.doi.org/10.1088/1367-2630/ab447a doi:10.1088/1367-2630/ab447a (ID: 720264) Toggle Abstract
  We perform quantum logic spectroscopy with a 27Al+/40Ca+ mixed ion crystal in a linear Paul trap for a measurement of the (3s2)1S0 to (3s3p)3P1,F=7/2 intercombination transition in 27Al+. Towards this end, Ramsey spectroscopy is used for probing the transition in 27Al+ and the (4s2)S1/2 to (4s3d)D5/2 clock transition in 40Ca+ in interleaved measurements. By using the precisely measured frequency of the clock transition in 40Ca+ as a frequency reference, we determine the frequency of the intercombination line to be 1S0 to 3P1,F=7/2=1122 842 857 334 736(93) Hz and the Landé g-factor of the excited state to be g3P1,F=7/2=0.428132(2).
• T. Brydges, A. Elben, P. Jurcevic, B. Vermersch, C. Maier, B. P. Lanyon, P. Zoller, R. Blatt, C. F. Roos Probing Renyi entanglement entropy via randomized measurements, Science 364 260 (2019-04-19), http://dx.doi.org/10.1126/science.aau4963 doi:10.1126/science.aau4963 (ID: 720034) Toggle Abstract
  Entanglement is the key feature of many-body quantum systems, and the development of new tools to probe it in the laboratory is an outstanding challenge. Measuring the entropy of different partitions of a quantum system provides a way to probe its entanglement structure. Here, we present and experimentally demonstrate a new protocol for measuring entropy, based on statistical correlations between randomized measurements. Our experiments, carried out with a trapped-ion quantum simulator, prove the overall coherent character of the system dynamics and reveal the growth of entanglement between its parts - both in the absence and presence of disorder. Our protocol represents a universal tool for probing and characterizing engineered quantum systems in the laboratory, applicable to arbitrary quantum states of up to several tens of qubits.
• M. Lee, K. Friebe, D. Fioretto, K. Schüppert, F. Ong, D. Plankensteiner, V. Torggler, H. Ritsch, R. Blatt, T. E. Northup Ion-Based Quantum Sensor for Optical Cavity Photon Numbers, Phys. Rev. Lett. 122 153603 (2019-04-19), http://dx.doi.org/10.1103/PhysRevLett.122.153603 doi:10.1103/PhysRevLett.122.153603 (ID: 720499) Toggle Abstract
  We dispersively couple a single trapped ion to an optical cavity to extract information about the cavity photon-number distribution in a nondestructive way. The photon-number-dependent ac Stark shift experienced by the ion is measured via Ramsey spectroscopy. We use these measurements first to obtain the ion-cavity interaction strength. Next, we reconstruct the cavity photon-number distribution for coherent states and for a state with mixed thermal-coherent statistics, finding overlaps above 99% with the calibrated states.
• C. Maier, T. Brydges, P. Jurcevic, N. Trautmann, C. Hempel, B. P. Lanyon, P. Hauke, R. Blatt, C. F. Roos Environment-assisted quantum transport in a 10-qubit network, Phys. Rev. Lett. 122 50501 (2019-02-08), http://dx.doi.org/10.1103/PhysRevLett.122.050501 doi:10.1103/PhysRevLett.122.050501 (ID: 720065) Toggle Abstract
  The way in which energy is transported through an interacting system governs fundamental properties in many areas of physics, chemistry, and biology. Remarkably, environmental noise can enhance the transport, an effect known as environment-assisted quantum transport (ENAQT). In this paper, we study ENAQT in a network of coupled spins subject to engineered static disorder and temporally varying dephasing noise. The interacting spin network is realized in a chain of trapped atomic ions and energy transport is represented by the transfer of electronic excitation between ions. With increasing noise strength, we observe a crossover from coherent dynamics and Anderson localization to ENAQT and finally a suppression of transport due to the quantum Zeno effect. We found that in the regime where ENAQT is most effective the transport is mainly diffusive, displaying coherences only at very short times. Further, we show that dephasing characterized by non-Markovian noise can maintain coherences longer than white noise dephasing, with a strong influence of the spectral structure on the transport effciency. Our approach represents a controlled and scalable way to investigate quantum transport in many-body networks under static disorder and dynamic noise.
• K. Lakhmanskiy, P. Holz, D. Schärtl, B. Ames, R. Assouly, T. Monz, Y. Colombe, R. Blatt Observation of superconductivity and surface noise using a single trapped ion as a field probe, Phys. Rev. A 99 23405 (2019-02-04), http://dx.doi.org/10.1103/PhysRevA.99.023405 doi:10.1103/PhysRevA.99.023405 (ID: 720500) Toggle Abstract
  Measuring and understanding electric-field noise from bulk material and surfaces is important for many areas of physics. In this work, we demonstrate the probing of electric-field noise from different sources with an ion, 225μm above the trap surface. We detect noise levels as small as SE=5.2(11)×10−16V2m−2Hz−1 at ωz=2π×1.51MHz and T=12K. Our setup incorporates a controllable noise source utilizing a high-temperature superconductor. This element allows us, first, to benchmark and validate the sensitivity of our probe. Second, it allows us to probe noninvasively the bulk properties of the superconductor, observing a superconducting transition with an ion. For temperatures below the transition, we use our setup to assess different surface noise processes. The measured surface noise shows a deviation from a power law in the frequency domain. However, the temperature scaling of the data is not in a good agreement with existing surface noise models. Our results open perspectives for models in surface science and pave the way to test them experimentally.

Report

• R. Blatt Excellence Strategy University of Heidelberg, (2019-03-17), (ID: 720364)

2018

Articles

• L. Postler, A. Rivas, P. Schindler, A. Erhardt, R. Stricker, D. Nigg, T. Monz, R. Blatt, M. Müller Experimental quantification of spatial correlations in quantum dynamics, Quantum 2 90 (2018-09-03), http://dx.doi.org/10.22331/q-2018-09-03-90 doi:10.22331/q-2018-09-03-90 (ID: 720164) Toggle Abstract
  Correlations between different partitions of quantum systems play a central role in a variety of many-body quantum systems, and they have been studied exhaustively in experimental and theoretical research. Here, we investigate dynamical correlations in the time evolution of multiple parts of a composite quantum system. A rigorous measure to quantify correlations in quantum dynamics based on a full tomographic reconstruction of the quantum process has been introduced recently [Á. Rivas et al., New Journal of Physics, 17(6) 062001 (2015).]. In this work, we derive a lower bound for this correlation measure, which does not require full knowledge of the quantum dynamics. Furthermore we also extend the correlation measure to multipartite systems. We directly apply the developed methods to a trapped ion quantum information processor to experimentally characterize the correlations in quantum dynamics for two- and four-qubit systems. The method proposed and demonstrated in this work is scalable, platform-independent and applicable to other composite quantum systems and quantum information processing architectures. We apply the method to estimate spatial correlations in environmental noise processes, which are crucial for the performance of quantum error correction procedures.
• C. Hempel, C. Maier, J. Romero, J. McClean, T. Monz, H. Shen, P. Jurcevic, B. P. Lanyon, P. Love, R. Babbush, A. Aspuru-Guzik, R. Blatt, C. F. Roos Quantum chemistry calculations on a trapped-ion quantum simulator, Phys. Rev. X 8 31022 (2018-07-24), http://dx.doi.org/10.1103/PhysRevX.8.031022 doi:10.1103/PhysRevX.8.031022 (ID: 720003) Toggle Abstract
  Quantum-classical hybrid algorithms are emerging as promising candidates for near-term practical applications of quantum information processors in a wide variety of fields ranging from chemistry to physics and materials science. We report on the experimental implementation of such an algorithm to solve a quantum chemistry problem, using a digital quantum simulator based on trapped ions. Specifically, we implement the variational quantum eigensolver algorithm to calculate the molecular ground state energies of two simple molecules and experimentally demonstrate and compare different encoding methods using up to four qubits. Furthermore, we discuss the impact of measurement noise as well as mitigation strategies and indicate the potential for adaptive implementations focused on reaching chemical accuracy, which may serve as a cross-platform benchmark for multi-qubit quantum simulators.
• G. Araneda, D. Higginbottom, L. Slodicka, Y. Colombe, R. Blatt Interference of Single Photons Emitted by Entangled Atoms in Free Space, Phys. Rev. Lett. 120 193603 (2018-05-11), http://dx.doi.org/10.1103/PhysRevLett.120.193603 doi:10.1103/PhysRevLett.120.193603 (ID: 720162) Toggle Abstract
  The generation and manipulation of entanglement between isolated particles has precipitated rapid progress in quantum information processing. Entanglement is also known to play an essential role in the optical properties of atomic ensembles, but fundamental effects in the controlled emission and absorption from small, well-defined numbers of entangled emitters in free space have remained unobserved. Here we present the control of the emission rate of a single photon from a pair of distant, entangled atoms into a free-space optical mode. Changing the length of the optical path connecting the atoms modulates the single-photon emission rate in the selected mode with a visibility V=0.27±0.03 determined by the degree of entanglement shared between the atoms, corresponding directly to the concurrence Cρ=0.31±0.10 of the prepared state. This scheme, together with population measurements, provides a fully optical determination of the amount of entanglement. Furthermore, large sensitivity of the interference phase evolution points to applications of the presented scheme in high-precision gradient sensing.
• N. Friis, O. Marty, C. Maier, C. Hempel, M. Holzapfel, P. Jurcevic, M. Plenio, M. Huber, C. F. Roos, R. Blatt, B. P. Lanyon Observation of Entangled States of a Fully Controlled 20-Qubit System, Phys. Rev. X 8 21012 (2018-04-10), http://dx.doi.org/10.1103/PhysRevX.8.021012 doi:10.1103/PhysRevX.8.021012 (ID: 720009) Toggle Abstract
  We generate and characterize entangled states of a register of 20 individually controlled qubits, where each qubit is encoded into the electronic state of a trapped atomic ion. Entanglement is generated amongst the qubits during the out-of-equilibrium dynamics of an Ising-type Hamiltonian, engineered via laser fields. Since the qubit-qubit interactions decay with distance, entanglement is generated at early times predominantly between neighboring groups of qubits. We characterize entanglement between these groups by designing and applying witnesses for genuine multipartite entanglement. Our results show that, during the dynamical evolution, all neighboring qubit pairs, triplets, most quadruplets, and some quintuplets simultaneously develop genuine multipartite entanglement. Witnessing genuine multipartite entanglement in larger groups of qubits in our system remains an open challenge.

2017

Articles

• A. Bermudez, X. Xu, R. Nigmatullin, J. O’Gorman, V. Negnevitsky, P. Schindler, T. Monz, U. Poschinger, C. Hempel, J. Home, F. Schmidt-Kaler, M. Biercuk, R. Blatt, S. Benjamin, M. Müller Assessing the Progress of Trapped-Ion Processors Towards Fault-Tolerant Quantum Computation, Phys. Rev. X 7 41061 (2017-12-17), http://dx.doi.org/10.1103/PhysRevX.7.041061 doi:10.1103/PhysRevX.7.041061 (ID: 719935) Toggle Abstract
  A quantitative assessment of the progress of small prototype quantum processors towards fault-tolerant quantum computation is a problem of current interest in experimental and theoretical quantum information science. We introduce a necessary and fair criterion for quantum error correction (QEC), which must be achieved in the development of these quantum processors before their sizes are sufficiently big to consider the well-known QEC threshold. We apply this criterion to benchmark the ongoing effort in implementing QEC with topological color codes using trapped-ion quantum processors and, more importantly, to guide the future hardware developments that will be required in order to demonstrate beneficial QEC with small topological quantum codes. In doing so, we present a thorough description of a realistic trapped-ion toolbox for QEC and a physically motivated error model that goes beyond standard simplifications in the QEC literature. We focus on laser-based quantum gates realized in two-species trapped-ion crystals in high-optical aperture segmented traps. Our large-scale numerical analysis shows that, with the foreseen technological improvements described here, this platform is a very promising candidate for fault-tolerant quantum computation.
• B. P. Lanyon, C. Maier, M. Holzapfel, T. Baumgratz, C. Hempel, P. Jurcevic, I. Dhand, A. S. Buyskik, A. J. Daley, M. Cramer, M. Plenio, R. Blatt, C. F. Roos Efficient tomography of a quantum many-body system, Nature Phys. 13 1158 (2017-12-05), http://dx.doi.org/10.1038/nphys4244 doi:10.1038/nphys4244 (ID: 719716) Toggle Abstract
  Quantum state tomography (QST) is the gold standard technique for obtaining an estimate for the state of small quantum systems in the laboratory. Its application to systems with more than a few constituents (e.g. particles) soon becomes impractical as the effort required grows exponentially in the number of constituents. Developing more efficient techniques is particularly pressing as precisely-controllable quantum systems that are well beyond the reach of QST are emerging in laboratories. Motivated by this, there is a considerable ongoing effort to develop new characterisation tools for quantum many-body systems. Here we demonstrate Matrix Product State (MPS) tomography, which is theoretically proven to allow the states of a broad class of quantum systems to be accurately estimated with an effort that increases efficiently with constituent number. We first prove that this broad class includes the out-of-equilbrium states produced by 1D systems with finite-range interactions, up to any fixed point in time. We then use the technique to reconstruct the dynamical state of a trapped-ion quantum simulator comprising up to 14 entangled spins (qubits): a size far beyond the reach of QST. Our results reveal the dynamical growth of entanglement and description complexity as correlations spread out during a quench: a necessary condition for future beyond-classical performance. MPS tomography should find widespread use to study large quantum many-body systems and to benchmark and verify quantum simulators and computers.
• A. Bermudez, P. Schindler, T. Monz, R. Blatt, M. Müller Micromotion-enabled improvement of quantum logic gates with trapped ions, New J. Phys. 19 113038 (2017-11-24), http://dx.doi.org/10.1088/1367-2630/aa86eb doi:10.1088/1367-2630/aa86eb (ID: 719937) Toggle Abstract
  The micromotion of ion crystals confined in Paul traps is usually considered an inconvenient nuisance, and is thus typically minimized in high-precision experiments such as high-fidelity quantum gates for quantum information processing (QIP). In this work, we introduce a particular scheme where this behavior can be reversed, making micromotion beneficial for QIP. We show that using laser-driven micromotion sidebands, it is possible to engineer state-dependent dipole forces with a reduced effect of off-resonant couplings to the carrier transition. This allows one, in a certain parameter regime, to devise entangling gate schemes based on geometric phase gates with both a higher speed and a lower error, which is attractive in light of current efforts towards fault-tolerant QIP. We discuss the prospects of reaching the parameters required to observe this micromotion-enabled improvement in experiments with current and future trap designs.
• C. A. Muschik, M. Heyl, E. A. Martínez, T. Monz, P. Schindler, B. Vogell, M. Dalmonte, P. Hauke, R. Blatt, P. Zoller U(1) Wilson lattice gauge theories in digital quantum simulators, New J. Phys. 19 103020 (2017-10-20), http://dx.doi.org/10.1088/1367-2630/aa89ab doi:10.1088/1367-2630/aa89ab (ID: 719936) Toggle Abstract
  Lattice gauge theories describe fundamental phenomena in nature, but calculating their real-time dynamics on classical computers is notoriously difficult. In a recent publication (Martinez et al 2016 Nature 534 516), we proposed and experimentally demonstrated a digital quantum simulation of the paradigmatic Schwinger model, a U(1)-Wilson lattice gauge theory describing the interplay between fermionic matter and gauge bosons. Here, we provide a detailed theoretical analysis of the performance and the potential of this protocol. Our strategy is based on analytically integrating out the gauge bosons, which preserves exact gauge invariance but results in complicated long-range interactions between the matter fields. Trapped-ion platforms are naturally suited to implementing these interactions, allowing for an efficient quantum simulation of the model, with a number of gate operations that scales polynomially with system size. Employing numerical simulations, we illustrate that relevant phenomena can be observed in larger experimental systems, using as an example the production of particle–antiparticle pairs after a quantum quench. We investigate theoretically the robustness of the scheme towards generic error sources, and show that near-future experiments can reach regimes where finite-size effects are insignificant. We also discuss the challenges in quantum simulating the continuum limit of the theory. Using our scheme, fundamental phenomena of lattice gauge theories can be probed using a broad set of experimentally accessible observables, including the entanglement entropy and the vacuum persistence amplitude.
• P. Jurcevic, H. Shen, P. Hauke, C. Maier, T. Brydges, C. Hempel, B. P. Lanyon, M. Heyl, R. Blatt, C. F. Roos Direct observation of dynamical quantum phase transitions in an interacting many-body system, Phys. Rev. Lett. 119 80501 (2017-08-21), http://dx.doi.org/10.1103/PhysRevLett.119.080501 doi:10.1103/PhysRevLett.119.080501 (ID: 719714) Toggle Abstract
  Dynamical quantum phase transitions (DQPTs) extend the concept of phase transitions and thus universality to the non-equilibrium regime. In this letter, we investigate DQPTs in a string of ions simulating interacting transverse-field Ising models. We observe non-equilibrium dynamics induced by a quantum quench and show for strings of up to 10 ions the direct detection of DQPTs by measuring a quantity that becomes non-analytic in time in the thermodynamic limit. Moreover, we provide a link between DQPTs and the dynamics of other relevant quantities such as the magnetization, and we establish a connection between DQPTs and entanglement production.
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• C. A. Riofrio, D. Gross, S. Flammia, T. Monz, D. Nigg, R. Blatt, J. Eisert Experimental quantum compressed sensing for a seven-qubit system, Nat. Commun. 8 15305 (2017-05-17), http://dx.doi.org/10.1038/ncomms1530 doi:10.1038/ncomms1530 (ID: 719934) Toggle Abstract
  Well-controlled quantum devices with their increasing system size face a new roadblock hindering further development of quantum technologies. The effort of quantum tomography—the reconstruction of states and processes of a quantum device—scales unfavourably: state-of-the-art systems can no longer be characterized. Quantum compressed sensing mitigates this problem by reconstructing states from incomplete data. Here we present an experimental implementation of compressed tomography of a seven-qubit system—a topological colour code prepared in a trapped ion architecture. We are in the highly incomplete—127 Pauli basis measurement settings—and highly noisy—100 repetitions each—regime. Originally, compressed sensing was advocated for states with few non-zero eigenvalues. We argue that low-rank estimates are appropriate in general since statistical noise enables reliable reconstruction of only the leading eigenvectors. The remaining eigenvectors behave consistently with a random-matrix model that carries no information about the true state.

2016

Articles

• E. A. Martínez, C. A. Muschik, P. Schindler, D. Nigg, A. Erhard, M. Heyl, P. Hauke, M. Dalmonte, T. Monz, P. Zoller, R. Blatt Real-time dynamics of lattice gauge theories with a few-qubit quantum computer, Nature 534 519 (2016-06-22), http://dx.doi.org/10.1038/nature18318 doi:10.1038/nature18318 (ID: 719563) Toggle Abstract
  Gauge theories are fundamental to our understanding of interactions between the elementary constituents of matter as mediated by gauge bosons. However, computing the real-time dynamics in gauge theories is a notorious challenge for classical computational methods. In the spirit of Feynman's vision of a quantum simulator, this has recently stimulated theoretical effort to devise schemes for simulating such theories on engineered quantum-mechanical devices, with the difficulty that gauge invariance and the associated local conservation laws (Gauss laws) need to be implemented. Here we report the first experimental demonstration of a digital quantum simulation of a lattice gauge theory, by realising 1+1-dimensional quantum electrodynamics (Schwinger model) on a few-qubit trapped-ion quantum computer. We are interested in the real-time evolution of the Schwinger mechanism, describing the instability of the bare vacuum due to quantum fluctuations, which manifests itself in the spontaneous creation of electron-positron pairs. To make efficient use of our quantum resources, we map the original problem to a spin model by eliminating the gauge fields in favour of exotic long-range interactions, which have a direct and efficient implementation on an ion trap architecture. We explore the Schwinger mechanism of particle-antiparticle generation by monitoring the mass production and the vacuum persistence amplitude. Moreover, we track the real-time evolution of entanglement in the system, which illustrates how particle creation and entanglement generation are directly related. Our work represents a first step towards quantum simulating high-energy theories with atomic physics experiments, the long-term vision being the extension to real-time quantum simulations of non-Abelian lattice gauge theories.
• M. F. Brandl, M. van Mourik, L. Postler, A. Nolf, K. Lakhmanskiy, R. R. Paiva, S. Möller, N. Daniilidis, H. Häffner, V. Kaushal, T. Ruster, C. Warschburger, H. Kaufmann, U. Poschinger, F. Schmidt-Kaler, P. Schindler, T. Monz, R. Blatt Cryogenic setup for trapped ion quantum computing, Review of Scientific Instruments (Online) 87 113103 (2016-06-21), http://dx.doi.org/10.1063/1.4966970 doi:10.1063/1.4966970 (ID: 720506)
• D. Higginbottom, L. Slodicka, G. Araneda, L. Lachman, S. N. Filippov, M. Hennrich, R. Blatt Pure single photons from a trapped atom source, New J. Phys. 18 93038 (2016-06-21), http://dx.doi.org/10.1088/1367-2630/18/9/093038 doi:10.1088/1367-2630/18/9/093038 (ID: 720507)
• E. A. Martínez, T. Monz, D. Nigg, P. Schindler, R. Blatt Compiling quantum algorithms for architectures with multi-qubit gates, New J. Phys. 18 63029 (2016-06-21), http://dx.doi.org/10.1088/1367-2630/18/6/063029 doi:10.1088/1367-2630/18/6/063029 (ID: 720509)
• M. F. Brandl, P. Schindler, T. Monz, R. Blatt Cryogenic resonator design for trapped ion experiments in Paul traps, Appl. Phys. B Las. Opt. 122 157 (2016-06-21), http://dx.doi.org/10.1007/s00340-016-6430-z doi:10.1007/s00340-016-6430-z (ID: 720510)
• M. Müller, A. Rivas, E. A. Martínez, D. Nigg, P. Schindler, T. Monz, R. Blatt, M. A. Martin-Delgado Iterative phase optimization of elementary quantum error correcting codes, Phys. Rev. X 6 31030 (2016-06-16), (ID: 720508)
• R. Lechner, C. Maier, C. Hempel, P. Jurcevic, B. P. Lanyon, T. Monz, M. Brownnutt, R. Blatt, C. F. Roos Electromagnetically-induced-transparency ground-state cooling of long ion strings, Phys. Rev. A 93 53401 (2016-03-18), http://dx.doi.org/10.1103/PhysRevA.93.053401 doi:10.1103/PhysRevA.93.053401 (ID: 719536) Toggle Abstract
  Electromagnetically-induced-transparency (EIT) cooling is a ground-state cooling technique for trapped particles. EIT offers a broader cooling range in frequency space compared to more established methods. In this work, we experimentally investigate EIT cooling in strings of trapped atomic ions. In strings of up to 18 ions, we demonstrate simultaneous ground-state cooling of all radial modes in under 1 ms. This is a particularly important capability in view of emerging quantum simulation experiments with large numbers of trapped ions. Our analysis of the EIT cooling dynamics is based on a technique enabling single-shot measurements of phonon numbers, by rapid adiabatic passage on a vibrational sideband of a narrow transition.
• D. Nigg, T. Monz, P. Schindler, E. A. Martínez, M. Hennrich, R. Blatt, M. F. Pusey, T. Rudolph, J. Barrett Can different quantum state vectors correspond to the same physical state? An experimental test, New J. Phys. 18 13007 (2016-03-04), http://dx.doi.org/10.1088/1367-2630/18/1/013007 doi:10.1088/1367-2630/18/1/013007 (ID: 719518) Toggle Abstract
  A century after the development of quantum theory, the interpretation of a quantum state is still discussed. If a physicist claims to have produced a system with a particular quantum state vector, does this represent directly a physical property of the system, or is the state vector merely a summary of the physicist's information about the system? Assume that a state vector corresponds to a probability distribution over possible values of an unknown physical or 'ontic' state. Then, a recent no-go theorem shows that distinct state vectors with overlapping distributions lead to predictions different from quantum theory. We report an experimental test of these predictions using trapped ions. Within experimental error, the results confirm quantum theory. We analyse which kinds of models are ruled out.
• M. Kumph, c. Henkel, P. Rabl, M. Brownnutt, R. Blatt Electric-field noise above a thin dielectric layer on metal electrodes, New J. Phys. 18 23020 (2016-03-04), http://dx.doi.org/10.1088/1367-2630/18/2/023020 doi:10.1088/1367-2630/18/2/023020 (ID: 719519) Toggle Abstract
  The electric-field noise above a layered structure composed of a planar metal electrode covered by a thin dielectric is evaluated and it is found that the dielectric film considerably increases the noise level, in proportion to its thickness. Importantly, even a thin (mono) layer of a low-loss dielectric can enhance the noise level by several orders of magnitude compared to the noise above a bare metal. Close to this layered surface, the power spectral density of the electric field varies with the inverse fourth power of the distance to the surface, rather than with the inverse square, as it would above a bare metal surface. Furthermore, compared to a clean metal, where the noise spectrum does not vary with frequency (in the radio-wave and microwave bands), the dielectric layer can generate electric-field noise which scales in inverse proportion to the frequency. For various realistic scenarios, the noise levels predicted from this model are comparable to those observed in trapped-ion experiments. Thus, these findings are of particular importance for the understanding and mitigation of unwanted heating and decoherence in miniaturized ion traps.
• M. Kumph, P. Holz, K. Langer, M. R. Meraner, M. Niedermayr, M. Brownnutt, R. Blatt Operation of a planar-electrode ion-trap array with adjustable RF electrodes, New J. Phys. 18 23047 (2016-03-04), http://dx.doi.org/10.1088/1367-2630/18/2/023047 doi:10.1088/1367-2630/18/2/023047 (ID: 719520) Toggle Abstract
  One path to realizing systems of trapped atomic ions suitable for large-scale quantum computing and simulation is to create a two-dimensional (2D) array of ion traps. Interactions between nearest-neighbouring ions could then be turned on and off by tuning the ions' relative positions and frequencies. We demonstrate and characterize the operation of a planar-electrode ion-trap array. By driving the trap with a network of phase-locked radio-frequency resonators which provide independently variable voltage amplitudes we vary the position and motional frequency of a Ca+ ion in two-dimensions within the trap array. Work on fabricating a miniaturised form of this 2D trap array is also described, which could ultimately provide a viable architecture for large-scale quantum simulations.
• T. Monz, D. Nigg, E. A. Martínez, M. F. Brandl, P. Schindler, R. Rines, S. Wang, I. L. Chuang, R. Blatt Realization of a scalable Shor algorithm, Science 351 1070 (2016-03-04), http://dx.doi.org/10.1126/science.aad9480 doi:10.1126/science.aad9480 (ID: 719521) Toggle Abstract
  Certain algorithms for quantum computers are able to outperform their classical counterparts. In 1994, Peter Shor came up with a quantum algorithm that calculates the prime factors of a large number vastly more efficiently than a classical computer. For general scalability of such algorithms, hardware, quantum error correction, and the algorithmic realization itself need to be extensible. Here we present the realization of a scalable Shor algorithm, as proposed by Kitaev. We factor the number 15 by effectively employing and controlling seven qubits and four “cache qubits” and by implementing generalized arithmetic operations, known as modular multipliers. This algorithm has been realized scalably within an ion-trap quantum computer and returns the correct factors with a confidence level exceeding 99%.

2015

Articles

• M. Brownnutt, M. Kumph, P. Rabl, R. Blatt Ion-trap measurements of electric-field noise near surfaces, Rev. Mod. Phys. 87 1419 (2015-12-22), http://dx.doi.org/10.1103/RevModPhys.87.1419 doi:10.1103/RevModPhys.87.1419 (ID: 719442) Toggle Abstract
  How can the electric noise in the vicinity of a metallic body be measured and understood? Trapped ions, known as unique tools for metrology and quantum information processing, also constitute very sensitive probes of this electric noise for distances from micrometers to millimeters. This paper presents various models for the origin of the electric noise, provides a critical review of the experimental findings, and summarizes the important questions that are still open in this active research area. Electric-field noise near surfaces is a common problem in diverse areas of physics and a limiting factor for many precision measurements. There are multiple mechanisms by which such noise is generated, many of which are poorly understood. Laser-cooled, trapped ions provide one of the most sensitive systems to probe electric-field noise at MHz frequencies and over a distance range 30−3000  μm from a surface. Over recent years numerous experiments have reported spectral densities of electric-field noise inferred from ion heating-rate measurements and several different theoretical explanations for the observed noise characteristics have been proposed. This paper provides an extensive summary and critical review of electric-field noise measurements in ion traps and compares these experimental findings with known and conjectured mechanisms for the origin of this noise. This reveals that the presence of multiple noise sources, as well as the different scalings added by geometrical considerations, complicates the interpretation of these results. It is thus the purpose of this review to assess which conclusions can be reasonably drawn from the existing data, and which important questions are still open. In so doing it provides a framework for future investigations of surface-noise processes.
• B. Casabone, K. Friebe, B. Brandstätter, K. Schüppert, R. Blatt, T. E. Northup Enhanced Quantum Interface with Collective Ion-Cavity Coupling, Phys. Rev. Lett. 114 023602 (2015-10-01), http://dx.doi.org/10.1103/PhysRevLett.114.023602 doi:10.1103/PhysRevLett.114.023602 (ID: 719349) Toggle Abstract
  We prepare a maximally entangled state of two ions and couple both ions to the mode of an optical cavity. The phase of the entangled state determines the collective interaction of the ions with the cavity mode, that is, whether the emission of a single photon into the cavity is suppressed or enhanced. By adjusting this phase, we tune the ion-cavity system from sub- to superradiance. We then encode a single qubit in the two-ion superradiant state and show that this encoding enhances the transfer of quantum information onto a photon.
• M. Guggemos, D. Heinrich, Ó. A. Herrera, R. Blatt, C. F. Roos Sympathetic cooling and detection of a hot trapped ion by a cold one, New J. Phys. 17 103001 (2015-07-29), http://dx.doi.org/10.1088/1367-2630/17/10/103001 doi:10.1088/1367-2630/17/10/103001 (ID: 719305) Toggle Abstract
  We investigate the dynamics of an ion sympathetically cooled by another laser-cooled ion or small ion crystal. To this end, we develop simple models of the cooling dynamics in the limit of weak Coulomb interactions. Experimentally, we create a two-ion crystal of Ca+ and Al+ by photo-ionization of neutral atoms produced by laser ablation. We characterize the velocity distribution of the laser-ablated atoms crossing the trap by time-resolved fluorescence spectroscopy. We observe neutral atom velocities much higher than the ones of thermally heated samples and find as a consequence long sympathethic cooling times before crystallization occurs. Our key result is a new technique for detecting the loading of an initially hot ion with energy in the eV range by monitoring the motional state of a Doppler-cooled ion already present in the trap. This technique not only detects the ion but also provides information about the dynamics of the sympathetic cooling process.
• P. Jurcevic, P. Hauke, C. Maier, C. Hempel, B. P. Lanyon, R. Blatt, C. F. Roos Spectroscopy of interacting quasiparticles in trapped ions, Phys. Rev. Lett. 115 100501 (2015-05-11), http://dx.doi.org/10.1103/PhysRevLett.115.100501 doi:10.1103/PhysRevLett.115.100501 (ID: 719241) Toggle Abstract
  The static and dynamic properties of many-body quantum systems are often well described by collective excitations, known as quasiparticles. Engineered quantum systems offer the opportunity to study such emergent phenomena in a precisely controlled and otherwise inaccessible way. We present a spectroscopic technique to study artificial quantum matter and use it for characterizing quasiparticles in a many-body system of trapped atomic ions. Our approach is to excite combinations of the system's fundamental quasiparticle eigenmodes, given by delocalised spin waves. By observing the dynamical response to superpositions of such eigenmodes, we extract the system dispersion relation, magnetic order, and even detect signatures of quasiparticle interactions. Our technique is not limited to trapped ions, and it is suitable for verifying quantum simulators by tuning them into regimes where the collective excitations have a simple form.

2014

Articles

• T. E. Northup, R. Blatt Quantum information transfer using photons, Nature Photon. 8 363 (2014-11-07), http://dx.doi.org/10.1038/nphoton.2014.53 doi:10.1038/nphoton.2014.53 (ID: 719036)
• D. Nigg, M. Müller, E. A. Martínez, P. Schindler, M. Hennrich, T. Monz, M. A. Martin-Delgado, R. Blatt Quantum computations on a topologically encoded qubit, Science 345 305 (2014-11-07), http://dx.doi.org/10.1126/science.1253742 doi:10.1126/science.1253742 (ID: 719037)
• P. Jurcevic, B. P. Lanyon, P. Hauke, C. Hempel, P. Zoller, R. Blatt, C. F. Roos Quasiparticle engineering and entanglement propagation in a quantum many-body system, Nature 511 202 (2014-07-10), http://dx.doi.org/10.1038/nature13461 doi:10.1038/nature13461 (ID: 718717) Toggle Abstract
  The key to explaining a wide range of quantum phenomena is understanding how entanglement propagates around many-body systems. Furthermore, the controlled distribution of entanglement is of fundamental importance for quantum communication and computation. In many situations, quasiparticles are the carriers of information around a quantum system and are expected to distribute entanglement in a fashion determined by the system interactions. Here we report on the observation of magnon quasiparticle dynamics in a one-dimensional many-body quantum system of trapped ions representing an Ising spin model. Using the ability to tune the effective interaction range, and to prepare and measure the quantum state at the individual particle level, we observe new quasiparticle phenomena. For the first time, we reveal the entanglement distributed by quasiparticles around a many-body system. Second, for long-range interactions we observe the divergence of quasiparticle velocity and breakdown of the light-cone picture that is valid for short-range interactions. Our results will allow experimental studies of a wide range of phenomena, such as quantum transport, thermalisation, localisation and entanglement growth, and represent a first step towards a new quantum-optical regime with on-demand quasiparticles with tunable non-linear interactions.
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• B. P. Lanyon, M. Zwerger, P. Jurcevic, C. Hempel, W. Dür, H. J. Briegel, R. Blatt, C. F. Roos Experimental Violation of Multipartite Bell Inequalities with Trapped Ions, Phys. Rev. Lett. 112 100403 (2014-03-13), http://dx.doi.org/10.1103/PhysRevLett.112.100403 doi:10.1103/PhysRevLett.112.100403 (ID: 718697) Toggle Abstract
  We report on the experimental violation of multipartite Bell inequalities by entangled states of trapped ions. First, we consider resource states for measurement-based quantum computation of between 3 and 7 ions and show that all strongly violate a Bell-type inequality for graph states, where the criterion for violation is a sufficiently high fidelity. Second, we analyze Greenberger-Horne-Zeilinger states of up to 14 ions generated in a previous experiment using stronger Mermin-Klyshko inequalities, and show that in this case the violation of local realism increases exponentially with system size. These experiments represent a violation of multipartite Bell-type inequalities of deterministically prepared entangled states. In addition, the detection loophole is closed.

Preprints

• M. Niedermayr, K. Lakhmanskiy, M. Kumph, S. Partel, J. Edlinger, M. Brownnutt, R. Blatt Cryogenic silicon surface ion trap, (2014-11-15), arXiv:1403.5208 arXiv:1403.5208 (ID: 719049) Toggle Abstract
  Trapped ions are pre-eminent candidates for building quantum information processors and quantum simulators. They have been used to demonstrate quantum gates and algorithms, quantum error correction, and basic quantum simulations. However, to realise the full potential of such systems and make scalable trapped-ion quantum computing a reality, there exist a number of practical problems which must be solved. These include tackling the observed high ion-heating rates and creating scalable trap structures which can be simply and reliably produced. Here, we report on cryogenically operated silicon ion traps which can be rapidly and easily fabricated using standard semiconductor technologies. Single 40Ca+ ions have been trapped and used to characterize the trap operation. Long ion lifetimes were observed with the traps exhibiting heating rates as low as d<n>/dt= 0.33 phonons/s at an ion-electrode distance of 230 μm. These results open many new avenues to arrays of micro-fabricated ion traps.
• M. Kumph, P. Holz, K. Langer, M. Niedermayr, M. Brownnutt, R. Blatt Operation of a planar-electrode ion trap array with adjustable RF electrodes, (2014-02-04), arXiv:1402.0791 arXiv:1402.0791 (ID: 719047) Toggle Abstract
  One path to scaling-up trapped atomic ions for large-scale quantum computing and simulation is to create a two-dimensional array of ion traps in close proximity to each other. A method to control the interactions between nearest neighboring ions is demonstrated and characterized here, using an adjustable radio-frequency (RF) electrode between trapping sites. A printed circuit board planar-electrode ion trap is demonstrated, trapping laser-cooled 40Ca+ ions. RF shuttling and secular-frequency adjustment are shown as a function of the power applied to the addressed RF electrode. The trapped ion's heating rate is measured via a fluorescence recooling method.

2013

Articles

• B. Brandstätter, A. McClung, K. Schüppert, B. Casabone, K. Friebe, A. Stute, P. O. Schmidt, C. Deutsch , J. Reichel, R. Blatt, T. E. Northup Integrated fiber-mirror ion trap for strong ion-cavity coupling, Review of Scientific Instruments (Print) 84 123104 (2013-12-17), http://dx.doi.org/10.1063/1.4838696 doi:10.1063/1.4838696 (ID: 718695) Toggle Abstract
  We present and characterize fiber mirrors and a miniaturized ion-trap design developed to integrate a fiber-based Fabry-Perot cavity (FFPC) with a linear Paul trap for use in cavity-QED experiments with trapped ions. Our fiber-mirror fabrication process not only enables the construction of FFPCs with small mode volumes, but also allows us to minimize the influence of the dielectric fiber mirrors on the trapped-ion pseudopotential. We discuss the effect of clipping losses for long FFPCs and the effect of angular and lateral displacements on the coupling efficiencies between cavity and fiber. Optical profilometry allows us to determine the radii of curvature and ellipticities of the fiber mirrors. From finesse measurements, we infer a single-atom cooperativity of up to 12 for FFPCs longer than 200 μm in length; comparison to cavities constructed with reference substrate mirrors produced in the same coating run indicates that our FFPCs have similar scattering losses. We characterize the birefringence of our fiber mirrors, finding that careful fiber-mirror selection enables us to construct FFPCs with degenerate polarization modes. As FFPCs are novel devices, we describe procedures developed for handling, aligning, and cleaning them. We discuss experiments to anneal fiber mirrors and explore the influence of the atmosphere under which annealing occurs on coating losses, finding that annealing under vacuum increases the losses for our reference substrate mirrors. X-ray photoelectron spectroscopy measurements indicate that these losses may be attributable to oxygen depletion in the mirror coating. Special design considerations enable us to introduce a FFPC into a trapped ion setup. Our unique linear Paul trap design provides clearance for such a cavity and is miniaturized to shield trapped ions from the dielectric fiber mirrors. We numerically calculate the trap potential in the absence of fibers. In the experiment additional electrodes can be used to compensate distortions of the potential due to the fibers. Home-built fiber feedthroughs connect the FFPC to external optics, and an integrated nanopositioning system affords the possibility of retracting or realigning the cavity without breaking vacuum.
• P. Schindler, D. Nigg, T. Monz, J. T. Barreiro, E. A. Martínez, S. Wang, S. Quint, M. F. Brandl, V. Nebendahl, C. F. Roos, M. Chwalla, M. Hennrich, R. Blatt A quantum information processor with trapped ions, New J. Phys. 15 123012 (2013-12-06), http://dx.doi.org/10.1088/1367-2630/15/12/123012 doi:10.1088/1367-2630/15/12/123012 (ID: 718676) Toggle Abstract
  Quantum computers hold the promise to solve certain problems exponentially faster than their classical counterparts. Trapped atomic ions are among the physical systems in which building such a computing device seems viable. In this work we present a small-scale quantum information processor based on a string of 40Ca+ ions confined in a macroscopic linear Paul trap. We review our set of operations which includes non-coherent operations allowing us to realize arbitrary Markovian processes. In order to build a larger quantum information processor it is mandatory to reduce the error rate of the available operations which is only possible if the physics of the noise processes is well understood. We identify the dominant noise sources in our system and discuss their effects on different algorithms. Finally we demonstrate how our entire set of operations can be used to facilitate the implementation of algorithms by examples of the quantum Fourier transform and the quantum order finding algorithm.
• B. P. Lanyon, P. Jurcevic, M. Zwerger, C. Hempel, E. A. Martínez, W. Dür, H. J. Briegel, R. Blatt, C. F. Roos Measurement-based quantum computation with trapped ions, Phys. Rev. Lett. 111 210501 (2013-11-19), http://dx.doi.org/10.1103/PhysRevLett.111.210501 doi:10.1103/PhysRevLett.111.210501 (ID: 718582) Toggle Abstract
  Measurement-based quantum computation represents a powerful and flexible framework for quantum information processing, based on the notion of entangled quantum states as computational resources. The most prominent application is the one-way quantum computer, with the cluster state as its universal resource. Here we demonstrate the principles of measurement-based quantum computation using deterministically generated cluster states, in a system of trapped calcium ions. First we implement a universal set of operations for quantum computing. Second we demonstrate a family of measurement-based quantum error correction codes and show their improved performance as the code length is increased. The methods presented can be directly scaled up to generate graph states of several tens of qubits.
• G. Hétet, L. Slodicka, N. Röck, R. Blatt Free-space read-out and control of single-ion dispersion using quantum interference, Phys. Rev. A 88 041804(R) (2013-10-24), http://dx.doi.org/10.1103/PhysRevA.88.041804 doi:10.1103/PhysRevA.88.041804 (ID: 718623) Toggle Abstract
  We perform a free-space measurement and control of the refractive index of a single trapped ion in the presence of quantum interference effects. The single atom refractive index is characterized by the Faraday rotation of a laser field tightly focused onto a trapped and laser-cooled barium ion. It is tuned using the internal ion state that is optically controlled via a V or a Λ scheme. Measurements of the phase shift associated with an electromagnetically induced transparency are then performed and the internal state on the qubit transition is read-out with a detection fidelity of (98±1)%.
• B. P. Lanyon, P. Jurcevic, C. Hempel, M. Gessner, V. Vedral, R. Blatt, C. F. Roos Experimental generation of quantum discord via noisy processes, Phys. Rev. Lett. 111 100504 (2013-09-06), http://dx.doi.org/10.1103/PhysRevLett.111.100504 doi:10.1103/PhysRevLett.111.100504 (ID: 718585) Toggle Abstract
  Quantum systems in mixed states can be unentangled and yet still non-classically correlated. These correlations can be quantified by the quantum discord and might provide a resource for quantum information processing tasks. By precisely controlling the interaction of two ionic-qubits with their environment, we investigate the capability of noise to generate discord. Firstly we show that noise acting only one quantum system can generate discord between two. States generated in this way are restricted in terms of the rank of their correlation matrix. Secondly we show that classically-correlated noise processes are capable of generating a much broader range of discordant states, with correlation matrices of any rank. Our results show that noise processes, prevalent in many physical systems, can automatically generate non-classical correlations and highlight fundamental differences between discord and entanglement.
• J. T. Barreiro, J. Bancal, P. Schindler, D. Nigg, M. Hennrich, T. Monz, N. Gisin, R. Blatt Demonstration of genuine multipartite entanglement with device-independent witnesses, Nature Phys. 9 562 (2013-08-04), http://dx.doi.org/10.1038/nphys2705 doi:10.1038/nphys2705 (ID: 718862) Toggle Abstract
  Entanglement in a quantum system can be demonstrated experimentally by performing the measurements prescribed by an appropriate entanglement witness. However, the unavoidable mismatch between the implementation of measurements in practical devices and their precise theoretical modelling generally results in the undesired possibility of false-positive entanglement detection. Such scenarios can be avoided by using the recently developed device-independent entanglement witnesses (DIEWs) for genuine multipartite entanglement. Similarly to Bell inequalities, the only assumption of DIEWs is that consistent measurements are performed locally on each subsystem. No precise description of the measurement devices is required. Here we report an experimental test of DIEWs on up to six entangled 40Ca+ ions. We also demonstrate genuine multipartite quantum nonlocality between up to six parties with the detection loophole closed.
• C. Hempel, B. P. Lanyon, P. Jurcevic, R. Gerritsma, R. Blatt, C. F. Roos Entanglement-enhanced detection of single-photon scattering events, Nature Photon. 7 633 (2013-07-30), http://dx.doi.org/10.1038/nphoton.2013.172 doi:10.1038/nphoton.2013.172 (ID: 718575) Toggle Abstract
  The ability to detect the interaction of light and matter at the single-particle level is becoming increasingly important for many areas of science and technology. The absorption or emission of a photon on a narrow transition of a trapped ion can be detected with near unit probability, thereby enabling the realization of ultra-precise ion clocks and quantum information processing applications. Extending this sensitivity to broad transitions is challenging due to the difficulty of detecting the rapid photon scattering events in this case. Here, we demonstrate a technique to detect the scattering of a single photon on a broad optical transition with high sensitivity. Our approach is to use an entangled state to amplify the tiny momentum kick an ion receives upon scattering a photon. The method should find applications in spectroscopy of atomic and molecular ions and quantum information processing.
• P. Schindler, M. Müller, D. Nigg, J. T. Barreiro, E. A. Martínez, M. Hennrich, T. Monz, S. Diehl, P. Zoller, R. Blatt Quantum simulation of open-system dynamical maps with trapped ions, Nature Phys. 9 367 (2013-05-19), http://dx.doi.org/10.1038/nphys2630 doi:10.1038/nphys2630 (ID: 718325) Toggle Abstract
  Dynamical maps describe general transformations of the state of a physical system, and their iteration can be interpreted as generating a discrete time evolution. Prime examples include classical nonlinear systems undergoing transitions to chaos. Quantum mechanical counterparts show intriguing phenomena such as dynamical localization on the single particle level. Here we extend the concept of dynamical maps to an open-system, many-particle context: We experimentally explore the stroboscopic dynamics of a complex many-body spin model by means of a universal quantum simulator using up to five ions. In particular, we generate long-range phase coherence of spin by an iteration of purely dissipative quantum maps. We also demonstrate the characteristics of competition between combined coherent and dissipative non-equilibrium evolution. This opens the door for studying many-particle non-equilibrium physics and associated dynamical phase transitions with no immediate counterpart in equilibrium condensed matter systems. An error detection and reduction toolbox that facilitates the faithful quantum simulation of larger systems is developed as a first step in this direction.
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• P. Bushev, G. Hétet, L. Slodicka, D. Rotter, M. A. Wilson, F. Schmidt-Kaler, J. Eschner, R. Blatt Shot-noise-limited monitoring and phase locking of the motion of a single trapped ion, Phys. Rev. Lett. 110 133602 (2013-03-27), http://dx.doi.org/10.1103/PhysRevLett.110.133602 doi:10.1103/PhysRevLett.110.133602 (ID: 718471) Toggle Abstract
  We perform a high-resolution real-time readout of the motion of a single trapped and laser-cooled Ba ion. By using an interferometric setup, we demonstrate a shot-noise-limited measurement of thermal oscillations with a resolution of 4 times the standard quantum limit.We apply the real-time monitoring for phase control of the ion motion through a feedback loop, suppressing the photon recoil-induced phase diffusion. Because of the spectral narrowing in the phase-locked mode, the coherent ion oscillation is measured with a resolution of about 0.3 times the standard quantum limit
• A. Stute, B. Casabone, B. Brandstätter, K. Friebe, T. E. Northup, R. Blatt Quantum-state transfer from an ion to a photon, Nature Photon. 7 222 (2013-02-27), http://dx.doi.org/10.1038/nphoton.2012.358 doi:10.1038/nphoton.2012.358 (ID: 718446)
• L. Slodicka, G. Hétet, N. Röck, P. Schindler, M. Hennrich, R. Blatt Atom-atom entanglement by single-photon detection, Phys. Rev. Lett. 110 083603 (2013-02-22), http://dx.doi.org/10.1103/PhysRevLett.110.083603 doi:10.1103/PhysRevLett.110.083603 (ID: 718441)
• P. Schindler, T. Monz, D. Nigg, J. T. Barreiro, E. A. Martínez, M. F. Brandl, M. Chwalla, M. Hennrich, R. Blatt Undoing a quantum measurement, Phys. Rev. Lett. 110 070403 (2013-02-14), http://dx.doi.org/10.1103/PhysRevLett.110.070403 doi:10.1103/PhysRevLett.110.070403 (ID: 718428)
• D. Nigg, J. T. Barreiro, P. Schindler, M. Mohseni, T. Monz, M. Chwalla, M. Hennrich, R. Blatt Experimental Characterization of Quantum Dynamics Through Many-Body Interactions, Phys. Rev. Lett. 110 040603 (2013-02-06), http://dx.doi.org/10.1103/PhysRevLett.110.060403 doi:10.1103/PhysRevLett.110.060403 (ID: 718422)

Issue

• R. Blatt, I. Bloch, I. J. Cirac, P. Zoller Quantum simulation - an exciting adventure, Ann. Phys. 525 (2013-11-04), http://dx.doi.org/10.1002/andp.201370100 doi:10.1002/andp.201370100 (ID: 718616)

2012

Articles

• E. A. Hinds, R. Blatt NOBEL 2012 Physics: Manipulating individual quantum systems, Nature 492 55 (2012-12-05), http://dx.doi.org/10.1038/492055a doi:10.1038/492055a (ID: 718370)
• D. Gandolfi, M. Niedermayr, M. Kumph, M. Brownnutt, R. Blatt Compact radio-frequency resonator for cryogenic ion traps, Review of Scientific Instruments (Print) 83 084705 (2012-11-01), http://dx.doi.org/10.1063/1.4737889 doi:10.1063/1.4737889 (ID: 718262) Toggle Abstract
  We report on the investigation and implementation of a lumped-component, radio-frequency resonator used in a cryogenic vacuum environment to drive an ion trap. The resonator was required to achieve the voltages necessary to trap (∼100 V), while dissipating very little power. Ultimately, for an input voltage of 1.35 V, a voltage gain of 100 was measured at 5.7 K, using a design which dissipated only 18 mW. The resonator operated at a frequency of 7.64 MHz and had a Q of 700. Single 40Ca+ ions were confined in a trap driven by this device, providing proof of successful resonator operation at low temperature.
• M. Brownnutt, M. Harlander, W. Hänsel, R. Blatt Spatially-resolved potential measurement with ion crystals, Appl. Phys. B Las. Opt. 107 1125 (2012-06-27), http://dx.doi.org/10.1007/s00340-012-5032-7 doi:10.1007/s00340-012-5032-7 (ID: 718115)
• A. Stute, B. Casabone, B. Brandstätter, D. Habicher, H. Barros, P. O. Schmidt, T. E. Northup, R. Blatt Toward an ion-photon quantum interface in an optical cavity, Appl. Phys. B Las. Opt. 107 1145 (2012-06-27), http://dx.doi.org/10.1007/s00340-011-4861-0 doi:10.1007/s00340-011-4861-0 (ID: 718116)
• A. Stute, B. Casabone, P. Schindler, T. Monz, P. O. Schmidt, B. Brandstätter, T. E. Northup, R. Blatt Tunable ion-photon entanglement in an optical cavity, Nature 485 482 (2012-05-24), http://dx.doi.org/10.1038/nature11120 doi:10.1038/nature11120 (ID: 718088)
• L. Slodicka, G. Hétet, N. Röck, S. Gerber, P. Schindler, M. Kumph, M. Hennrich, R. Blatt Interferometric Thermometry of a single sub-Doppler-cooled atom, Phys. Rev. A 85 043401 (2012-04-05), http://dx.doi.org/10.1103/PhysRevA.85.043401 doi:10.1103/PhysRevA.85.043401 (ID: 718043) Toggle Abstract
  Efficient self-interference of single photons emitted by a sideband-cooled barium ion is demonstrated. First, the technical tools for performing efficient coupling to the quadrupolar transition of a single 138Ba+ ion are presented.We show efficient Rabi oscillations of the internal state of the ion by using a highly stabilized 1.76-μm fiber laser resonant with the S1/2-D5/2 transition. We then show sideband cooling of the ion’s motional modes and use it as a means to enhance the interference contrast of the ion with its mirror image to up to 90%. Last, we measure the dependence of the self-interference contrast on the mean phonon number, thereby demonstrating the potential of the setup for single-atom thermometry close to the motional ground state.
• R. Blatt, C. F. Roos Quantum simulations with trapped ions, Nature Phys. 8 277 (2012-04-03), http://dx.doi.org/10.1038/NPHYS2252 doi:10.1038/NPHYS2252 (ID: 718042)

2011

Articles

• G. Hétet, L. Slodicka, M. Hennrich, R. Blatt Single atom as a mirror of an optical cavity, Phys. Rev. Lett. 107 133002 (2011-09-20), http://dx.doi.org/10.1103/PhysRevLett.107.133002 doi:10.1103/PhysRevLett.107.133002 (ID: 717769) Toggle Abstract
  By tightly focussing a laser field onto a single cold ion trapped in front of a far-distant dielectric mirror, we could observe a quantum electrodynamic effect whereby the ion behaves as the optical mirror of a Fabry-P\'erot cavity. We show that the amplitude of the laser field is significantly altered due to a modification of the electromagnetic mode structure around the atom in a novel regime in which the laser intensity is already changed by the atom alone. e propose a direct application of this system as a quantum memory for single photons.
• B. P. Lanyon, C. Hempel, D. Nigg, M. Müller, R. Gerritsma, F. Zähringer, P. Schindler, J. T. Barreiro, M. Rambach, G. Kirchmair, M. Hennrich, P. Zoller, R. Blatt, C. F. Roos Universal Digital Quantum Simulation with Trapped Ions, Science 334 57 (2011-09-01), http://dx.doi.org/10.1126/science.1208001 doi:10.1126/science.1208001 (ID: 717768) Toggle Abstract
  A digital quantum simulator is an envisioned quantum device that can be programmed to efficiently simulate any other local system. We demonstrate and investigate the digital approach to quantum simulation in a system of trapped ions. Using sequences of up to 100 gates and 6 qubits, the full-time dynamics of a range of spin systems are digitally simulated. Interactions beyond those naturally present in our simulator are accurately reproduced, and quantitative bounds are provided for the overall simulation quality. Our results demonstrate the key principles of digital quantum simulation and provide evidence that the level of control required for a full-scale device is within reach.
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• M. Kumph, M. Brownnutt, R. Blatt Two-dimensional arrays of radio-frequency ion traps with addressable interactions, New J. Phys. 13 073043 (2011-08-25), http://dx.doi.org/10.1088/1367-2630/13/7/073043 doi:10.1088/1367-2630/13/7/073043 (ID: 717756) Toggle Abstract
  We describe the advantages of two-dimensional (2D), addressable arrays of spherical Paul traps. They would provide the ability to address and tailor the interaction strengths of trapped objects in 2D and could be a valuable new tool for quantum information processing. Simulations of trapping ions are compared to first tests utilizing printed circuit board trap arrays loaded with dust particles. Pair-wise interactions in the array are addressed by means of an adjustable radio-frequency (RF) electrode shared between trapping sites. By attenuating this RF electrode potential, neighboring pairs of trapped objects have their interaction strength increased and are moved closer to one another. In the limit of the adjustable electrode being held at RF ground, the two formerly spherical traps are merged into one linear Paul trap.
• R. Blatt Spin flips of a single proton, Nature 475 298 (2011-07-31), http://dx.doi.org/10.1038/475298a doi:10.1038/475298a (ID: 717908)
• P. Schindler, J. T. Barreiro, T. Monz, V. Nebendahl, D. Nigg, M. Chwalla, M. Hennrich, R. Blatt Experimental repetitive quantum error correction, Science 332 1059 (2011-05-27), http://dx.doi.org/10.1126/science.1203329 doi:10.1126/science.1203329 (ID: 717685)
• M. Harlander, R. Lechner, M. Brownnutt, R. Blatt, W. Hänsel Trapped-ion antennae for the transmission of quantum information, Nature 471 203 (2011-04-05), http://dx.doi.org/10.1038/nature09800 doi:10.1038/nature09800 (ID: 717659) Toggle Abstract
  More than one hundred years ago Heinrich Hertz succeeded in transmitting signals over a few meters to a receiving antenna using an electromagnetic oscillator and thus proving the electromagnetic theory developed by James C. Maxwell[1]. Since then, technology has developed, and today a variety of oscillators is available at the quantum mechanical level. For quantized electromagnetic oscillations atoms in cavities can be used to couple electric fields[2, 3]. For mechanical oscillators realized, for example, with cantilevers[4, 5] or vibrational modes of trapped atoms[6] or ions[7, 8], a quantum mechanical link between two such oscillators has, to date, been demonstrated in very few cases and has only been achieved in indirect ways. Examples of this include the mechanical transport of atoms carrying the quantum information[9] or the use of spontaneously emitted photons[10]. In this work, direct coupling between the motional dipoles of separately trapped ions is achieved over a distance of 54 {\mu}m, using the dipole-dipole interaction as a quantum-mechanical transmission line[11]. This interaction is small between single trapped ions, but the coupling is amplified by using additional trapped ions as antennae. With three ions in each well the interaction is increased by a factor of seven as compared to the singleion case. This enhancement facilitates bridging of larger distances and relaxes the constraints on the miniaturization of trap electrodes. This represents a new building block for quantum computation and also offers new opportunities to couple quantum systems of different natures.
• T. Monz, P. Schindler, J. T. Barreiro, M. Chwalla, D. Nigg, B. Coish, M. Harlander, W. Hänsel, M. Hennrich, R. Blatt 14-qubit entanglement: creation and coherence, Phys. Rev. Lett. 106 130506 (2011-04-05), http://dx.doi.org/10.1103/PhysRevLett.106.130506 doi:10.1103/PhysRevLett.106.130506 (ID: 717660) Toggle Abstract
  We report the creation of Greenberger-Horne-Zeilinger states with up to 14 qubits. By investigating the coherence of up to 8 ions over time, we observe a decay proportional to the square of the number of qubits. The observed decay agrees with a theoretical model which assumes a system affected by correlated, Gaussian phase noise. This model holds for the majority of current experimental systems developed towards quantum computation and quantum metrology.
• J. T. Barreiro, M. Müller, P. Schindler, D. Nigg, T. Monz, M. Chwalla, M. Hennrich, C. F. Roos, P. Zoller, R. Blatt An open-system quantum simulator with trapped ions, Nature 470 491 (2011-02-24), http://dx.doi.org/10.1038/nature09801 doi:10.1038/nature09801 (ID: 717617) Toggle Abstract
  The control of quantum systems is of fundamental scientific interest and promises powerful applications and technologies. Impressive progress has been achieved in isolating quantum systems from the environment and coherently controlling their dynamics, as demonstrated by the creation and manipulation of entanglement in various physical systems. However, for open quantum systems, engineering the dynamics of many particles by a controlled coupling to an environment remains largely unexplored. Here we realize an experimental toolbox for simulating an open quantum system with up to five quantum bits (qubits). Using a quantum computing architecture with trapped ions, we combine multi-qubit gates with optical pumping to implement coherent operations and dissipative processes. We illustrate our ability to engineer the open-system dynamics through the dissipative preparation of entangled states, the simulation of coherent many-body spin interactions, and the quantum non-demolition measurement of multi-qubit observables. By adding controlled dissipation to coherent operations, this work offers novel prospects for open-system quantum simulation and computation.
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• R. Gerritsma, B. P. Lanyon, G. Kirchmair, F. Zähringer, C. Hempel, J. Casanova, J. J. García-Ripoll, E. Solano, R. Blatt, C. F. Roos Quantum simulation of the Klein paradox with trapped ions, Phys. Rev. Lett. 106 060503 (2011-02-11), http://dx.doi.org/10.1103/PhysRevLett.106.060503 doi:10.1103/PhysRevLett.106.060503 (ID: 717611) Toggle Abstract
  We report on quantum simulations of relativistic scattering dynamics using trapped ions. The simulated state of a scattering particle is encoded in both the electronic and vibrational state of an ion, representing the discrete and continuous components of relativistic wave functions. Multiple laser fields and an auxiliary ion simulate the dynamics generated by the Dirac equation in the presence of a scattering potential. Measurement and reconstruction of the particle wave packet enables a frame-by-frame visualization of the scattering processes. By precisely engineering a range of external potentials we are able to simulate text book relativistic scattering experiments and study Klein tunneling in an analogue quantum simulator. We describe extensions to solve problems that are beyond current classical computing capabilities.

Proceeding

• C. F. Roos, R. Gerritsma, G. Kirchmair, F. Zähringer, E. Solano, R. Blatt Quantum simulation of relativistic quantum physics with trapped ions, 22nd International Conference on Atomic Physics (Cairns, Australia, 2010-07-25) J. Phys. Conf. Ser. 264 012020 (2011-01-27), http://dx.doi.org/10.1088/1742-6596/264/1/012020 doi:10.1088/1742-6596/264/1/012020 (ID: 717471)

2010

Articles

• G. Hétet, L. Slodicka, A. Glätzle, M. Hennrich, R. Blatt QED with a spherical mirror, Phys. Rev. A 063812 (2010-12-09), http://dx.doi.org/10.1103/PhysRevA.82.063812 doi:10.1103/PhysRevA.82.063812 (ID: 717398) Toggle Abstract
  We investigate the quantum electrodynamic (QED) properties of an atomic electron close to the focus of a spherical mirror. We first show that the spontaneous emission and excited-state level shift of the atom can be fully suppressed with mirror-atom distances of many wavelengths. A three-dimensional theory predicts that the spectral density of vacuum fluctuations can indeed vanish within a volume λ3 around the atom, with the use of a far-distant mirror covering only half of the atomic emission solid angle. The modification of these QED atomic properties is also computed as a function of the mirror size, and large effects are found for only moderate numerical apertures. We also evaluate the long-distance ground-state energy shift (Casimir-Polder shift) and find that it scales as (λ/R)2 at the focus of a hemispherical mirror of radius R, as opposed to the well-known (λ/R)4 scaling law for an atom at a distance R from an infinite plane mirror. Our results are relevant for investigations of QED effects as well as free-space coupling to single atoms using high-numerical-aperture lenses.
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• J. T. Barreiro, P. Schindler, O. Gühne, T. Monz, M. Chwalla, C. F. Roos, M. Hennrich, R. Blatt Experimental multiparticle entanglement dynamics induced by decoherence, Nature Phys. 6 943 (2010-12-02), http://dx.doi.org/10.1038/nphys1781 doi:10.1038/nphys1781 (ID: 717355) Toggle Abstract
  Multiparticle entanglement leads to richer correlations than two-particle entanglement and gives rise to striking contradictions with local realism1, inequivalent classes of entanglement2 and applications such as one-way or topological quantum computing3, 4. When exposed to decohering or dissipative environments, multiparticle entanglement yields subtle dynamical features and access to new classes of states and applications. Here, using a string of trapped ions, we experimentally characterize the dynamics of entanglement of a multiparticle state under the influence of decoherence. By embedding an entangled state of four qubits in a decohering environment (through spontaneous decay), we observe a rich dynamics crossing distinctive domains: Bell-inequality violation, entanglement superactivation, bound entanglement and full separability. We also develop new theoretical tools for characterizing entanglement in quantum states. Recent quantum-computing, state-engineering and simulation paradigms driven by dissipative or decohering environments5, 6, 7 can benefit from the environment engineering techniques demonstrated here.
• M. Harlander, M. Brownnutt, R. Blatt Trapped-ion probing of light-induced charging effects on dielectrics, 12 093035 (2010-10-06), http://dx.doi.org/10.1088/1367-2630/12/9/093035 doi:10.1088/1367-2630/12/9/093035 (ID: 717314) Toggle Abstract
  We use a string of confined 40Ca+ ions to measure perturbations to a trapping potential which are caused by the light-induced charging of an antireflection-coated window and of insulating patches on the ion-trap electrodes. The electric fields induced at the ions' position are characterized as a function of distance to the dielectric and as a function of the incident optical power and wavelength. The measurement of the ion-string position is sensitive to as few as 40 elementary charges per \sqrt{\rm Hz} on the dielectric at distances of the order of millimetres, and perturbations are observed for illuminations with light of wavelengths as large as 729 nm. This has important implications for the future of miniaturized ion-trap experiments, notably with regard to the choice of electrode material and the optics that must be integrated in the vicinity of the ion. The method presented here can be readily applied to the investigation of charging effects beyond the context of ion-trap experiments.
• T. Monz, P. Schindler, J. T. Barreiro, M. Chwalla, D. Nigg, B. Coish, M. Harlander, W. Hänsel, M. Hennrich, R. Blatt Electromagnetically Induced Transparency from a Single Atom in Free Space, Phys. Rev. Lett. 105 153604 (2010-10-06), http://dx.doi.org/10.1103/PhysRevLett.105.153604 doi:10.1103/PhysRevLett.105.153604 (ID: 717316) Toggle Abstract
  In this Letter, we report an absorption spectroscopy experiment and the observation of electromagnetically induced transparency from a single trapped atom. We focus a weak and narrow band Gaussian light beam onto an optically cooled 138Baþ ion using a high numerical aperture lens. Extinction of this beam is observed with measured values of up to 1.35%.We demonstrate electromagnetically induced transparency of the ion by tuning a strong control beam over a two-photon resonance in a three-level Lambda-type system. The probe beam extinction is inhibited by more than 75% due to population trapping.
• F. Dubin, C. Russo, H. Barros, A. Stute, C. Becher, P. O. Schmidt, R. Blatt Quantum to classical transition in a single-ion laser, Nature Phys. 6 350 (2010-03-29), http://dx.doi.org/10.1038/NPHYS1627 doi:10.1038/NPHYS1627 (ID: 717177)
• A. Glätzle, K. Hammerer, A. J. Daley, R. Blatt, P. Zoller A single trapped atom in front of an oscillating mirror, Opt. Com. 283 765 (2010-03-15), http://dx.doi.org/10.1016/j.optcom.2009.10.063 doi:10.1016/j.optcom.2009.10.063 (ID: 717009) Toggle Abstract
  We investigate the Wigner–Weisskopf decay of a two-level atom in front of an oscillating mirror. This work builds on and extends previous theoretical and experimental studies of the effects of a static mirror on spontaneous decay and resonance fluorescence. The spontaneously emitted field is inherently non-stationary due to the time-dependent boundary conditions and in order to study its spectral distribution we employ the operational definition of the spectrum of non-stationary light due to the seminal work by Eberly and Wódkiewicz. We find a rich dependence of this spectrum as well as of the effective decay rates and level shifts on the mirror–atom distance and on the amplitude and frequency of the mirror’s oscillations. The results presented here provide the basis for future studies of more complex setups, where the motion of the atom and/or the mirror are included as quantum degrees of freedom.
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• F. Zähringer, G. Kirchmair, R. Gerritsma, E. Solano, R. Blatt, C. F. Roos Realization of a Quantum Walk with One and Two Trapped Ions, Phys. Rev. Lett. 104 100503 (2010-03-09), http://dx.doi.org/10.1103/PhysRevLett.104.100503 doi:10.1103/PhysRevLett.104.100503 (ID: 717148)
• R. Gerritsma, G. Kirchmair, F. Zähringer, E. Solano, R. Blatt, C. F. Roos Quantum simulation of the Dirac equation, Nature 463 71 (2010-01-07), http://dx.doi.org/10.1038/nature08688 doi:10.1038/nature08688 (ID: 716977)

2009

Articles

• T. Monz, K. Kim, A. Villar, P. Schindler, M. Chwalla, M. Riebe, C. F. Roos, H. Häffner, W. Hänsel, M. Hennrich, R. Blatt Realization of universal ion-trap quantum computation with decoherence-free qubits, Phys. Rev. Lett. 103 200503 (2009-11-13), http://dx.doi.org/10.1103/PhysRevLett.103.200503 doi:10.1103/PhysRevLett.103.200503 (ID: 716801)
• M. Chwalla, J. Benhelm, K. Kim, G. Kirchmair, T. Monz, M. Riebe, P. Schindler, A. Villar, W. Hänsel, C. Roos, R. Blatt, M. Abgrall, G. Santarelli, G. D. Rovera, P. Laurent Absolute frequency measurement of the 40Ca+ 4s 2S1/2 - 3d 2D5/2 clock transition, Phys. Rev. Lett. 102 023002 (2009), http://dx.doi.org/10.1103/PhysRevLett.102.023002 doi:10.1103/PhysRevLett.102.023002 (ID: 650530)
• S. Gerber, D. Rotter, M. Hennrich, R. Blatt, F. Rohde, C. Schuck, M. Almendros, R. Gehr, F. Dubin, J. Eschner Quantum interference from remotely trapped ions, 11 013032 (2009), http://dx.doi.org/10.1088/1367-2630/11/1/013032 doi:10.1088/1367-2630/11/1/013032 (ID: 651475)
• T. Monz, K. Kim, W. Hänsel, M. Riebe, A. Villar, P. Schindler, M. Chwalla, M. Hennrich, R. Blatt Realization of the quantum Toffoli gate with trapped ions, Phys. Rev. Lett. 102 040501 (2009), http://dx.doi.org/10.1103/PhysRevLett.102.040501 doi:10.1103/PhysRevLett.102.040501 (ID: 654575)
• G. Kirchmair, J. Benhelm, F. Zähringer, R. Gerritsma, C. Roos, R. Blatt Deterministic entanglement of ions in thermal states of motion, 11 023002 (2009), http://dx.doi.org/10.1088/1367-2630/11/2/023002 doi:10.1088/1367-2630/11/2/023002 (ID: 655090)
• G. Kirchmair, J. Benhelm, F. Zähringer, R. Gerritsma, C. Roos, R. Blatt High-fidelity entanglement of 43Ca+ hyperfine clock states, Phys. Rev. A 79 020304 (2009), http://dx.doi.org/10.1103/PhysRevA.79.020304 doi:10.1103/PhysRevA.79.020304 (ID: 657856)
• C. Russo, H. Barros, A. Stute, F. Dubin, E. S. Philips, T. Monz, T. Northup, C. Becher, T. Salzburger, H. Ritsch, P. O. Schmidt, R. Blatt Raman spectroscopy of a single ion coupled to a high-finesse cavity, 95 205 (2009), http://dx.doi.org/10.1007/s00340-009-3430-2 doi:10.1007/s00340-009-3430-2 (ID: 674888)
• S. Gerber, D. Rotter, L. Slodicka, J. Eschner, H. J. Carmichael, R. Blatt Intensity-Field Correlation of Single-Atom Resonance Fluorescence, Phys. Rev. Lett. 102 183601 (2009), http://dx.doi.org/10.1103/PhysRevLett.102.183601 doi:10.1103/PhysRevLett.102.183601 (ID: 679866)
• G. Kirchmair, F. Zähringer, R. Gerritsma, M. Kleinmann, O. Gühne, A. Cabello, R. Blatt, C. Roos State-independent experimental test of quantum contextuality, Nature 460 494 (2009), http://dx.doi.org/10.1038/nature08172 doi:10.1038/nature08172 (ID: 695207) Toggle Abstract
  The question of whether quantum phenomena can be explained by classical models with hidden variables is the subject of a long-lasting debate. In 1964, Bell showed that certain types of classical models cannot explain the quantum mechanical predictions for specific states of distant particles, and some types of hidden variable models have been experimentally ruled out. An intuitive feature of classical models is non-contextuality: the property that any measurement has a value independent of other compatible measurements being carried out at the same time. However, a theorem derived by Kochen, Specker and Bell shows that non-contextuality is in conflict with quantum mechanics. The conflict resides in the structure of the theory and is independent of the properties of special states. It has been debated whether the Kochen–Specker theorem could be experimentally tested at all. First tests of quantum contextuality have been proposed only recently, and undertaken with photons and neutrons. But these tests required the generation of special quantum states and left various loopholes open. Here we perform an experiment with trapped ions that demonstrates a state-independent conflict with non-contextuality. The experiment is not subject to the detection loophole and we show that, despite imperfections and possible measurement disturbances, our results cannot be explained in non-contextual terms.
• H. Barros, A. Stute, T. Northup, C. Russo, P. O. Schmidt, R. Blatt Deterministic single-photon source from a single ion, 11 103004 (2009), http://dx.doi.org/10.1088/1367-2630/11/10/103004 doi:10.1088/1367-2630/11/10/103004 (ID: 716750)
• F. Splatt, M. Harlander, M. Brownnutt, F. Zähringer, R. Blatt, W. Hänsel Deterministic reordering of 40Ca+ ions in a linear segmented Paul trap, 11 103008 (2009), http://dx.doi.org/10.1088/1367-2630/11/10/103008 doi:10.1088/1367-2630/11/10/103008 (ID: 716755)

Proceedings

• M. Chwalla, J. Benhelm, K. Kim, G. Kirchmair, T. Monz, M. Riebe, P. Schindler, A. Villar, W. Hänsel, C. F. Roos, R. Blatt, M. Abgrall, G. Santarelli, D. Rovera, P. Laurent Absolute frequency measurement of the 40Ca+ S1/2-D5/2 clock transition, Proceedings of the 7th symposium on frequency standards and metrology 7th symposium on frequency standards and metrology (Pacific Grove, 2008-10-05) 108 (2009-10-28), (ID: 716779)
• J. Benhelm, G. Kirchmair, R. Gerritsma, F. Zähringer, T. Monz, P. Schindler, M. Chwalla, W. Hänsel, M. Hennrich, C. F. Roos, R. Blatt Ca+ quantum bits for quantum information processing, Physica Scripta T137 014008 (2009-05-25), (ID: 716979)

2008

Articles

• T. Calarco, M. Aspelmeyer, D. Binosi, R. Blatt, A. Zeilinger, E. Polzik A quantum leap in computing power, Public Service Review: European Union 439 (2008), (ID: 574597)
• D. Rotter, M. Mukherjee, F. Dubin, R. Blatt Monitoring a single ion's motion via second-order photon correlations, 10 043011 (2008), (ID: 576252)
• J. Benhelm, G. Kirchmair, C. Roos, R. Blatt Towards fault-tolerant quantum computing with trapped ions, Nature Phys. 4 463 (2008), http://dx.doi.org/10.1038/nphys961 doi:10.1038/nphys961 (ID: 578533)
• C. Roos, T. Monz, K. Kim, M. Riebe, H. Häffner, D. James, R. Blatt Nonlinear coupling of continuous variables at the single quantum level, Phys. Rev. A 77 040302(R) (2008), http://dx.doi.org/10.1103/PhysRevA.77.040302 doi:10.1103/PhysRevA.77.040302 (ID: 579025)
• K. Kim, C. Roos, L. Aolita, H. Häffner, V. Nebendahl, R. Blatt Geometric phase gate on an optical transition for ion trap quantum computation, Phys. Rev. A 77 050303(R) (2008), http://dx.doi.org/10.1103/PhysRevA.77.050303 doi:10.1103/PhysRevA.77.050303 (ID: 583053)
• J. Benhelm, G. Kirchmair, C. Roos, R. Blatt Experimental quantum information processing with 43Ca+ ions, Phys. Rev. A 77 062306 (2008), http://dx.doi.org/10.1103/PhysRevA.77.062306 doi:10.1103/PhysRevA.77.062306 (ID: 595124)
• R. Blatt, D. Wineland Entangled states of trapped atomic ions, Nature 453 1008 (2008), http://dx.doi.org/10.1038/nature07125 doi:10.1038/nature07125 (ID: 597200)
• R. Gerritsma, G. Kirchmair, F. Zähringer, J. Benhelm, R. Blatt, C. Roos Precision measurement of the branching fractions of the 4p 2P3/2 decay of Ca II, Eur. Phys. J. D 50 13 (2008), http://dx.doi.org/10.1140/epjd/e2008-00196-9 doi:10.1140/epjd/e2008-00196-9 (ID: 621123)
• M. Riebe, T. Monz, A. Villar, P. Schindler, M. Chwalla, M. Hennrich, R. Blatt Deterministic entanglement swapping with an ion trap quantum computer, Nature Phys. 4 839 (2008), (ID: 630215)
• H. Häffner, C. Roos, R. Blatt Quantum computing with trapped ions, Physics Reports 469 155-203 (2008), http://dx.doi.org/10.1016/j.physrep.2008.09.003 doi:10.1016/j.physrep.2008.09.003 (ID: 630261)

Proceedings

• R. Blatt, J. F. Poyatos, J. I. Cirac, P. Zoller "Quantum motion of a trapped ion: Schrödinger cats, ion interferometry and quantum tomography" in Proceedings of the 5th Symposium on Frequency Standards and Metrology, 20 (2008-10-11), (ID: 628467)
• C. Roos, M. Chwalla, T. Monz, P. Schindler, K. Kim, M. Riebe, R. Blatt Quantum information processing and Ramsey spectroscopy with trapped ions, Laser Spectroscopy: Proceedings of the XVIII International Conference (ICOLS) (Telluride, USA, 2007-06-24) 53 (2008), (ID: 594009)
• R. Blatt The Quantum Revolution - Towards a new generation of Supercomputers, Laser Spectroscopy: Proceedings of the XVIII International Conference (ICOLS) (Telluride, USA, 2007-06-24) 207 (2008), (ID: 594010)

2007

Articles

• C. Wunderlich, T. Hannemann, T. Körber, H. Häffner, C. Roos, W. Hänsel, R. Blatt, F. Schmidt-Kaler Robust state preparation of a single trapped ion by adiabatic passage, J. Mod. Opt. 1541 (2007), http://dx.doi.org/10.1080/09500340600741082 doi:10.1080/09500340600741082 (ID: 434966) (local copy)
• J. Benhelm, G. Kirchmair, U. D. Rapol, T. Körber, C. Roos, R. Blatt Measurement of the hyperfine structure of the S1/2-D5/2 transition in 43Ca+, Phys. Rev. A 75 032506 (2007), http://dx.doi.org/10.1103/PhysRevA.75.032506 doi:10.1103/PhysRevA.75.032506 (ID: 461034) (local copy)
• F. Dubin, D. Rotter, M. Mukherjee, C. Russo, J. Eschner, R. Blatt Photon Correlation versus Interference of Single-Atom Fluorescence in a Half-Cavity, Phys. Rev. Lett. 98 183003 (2007), (ID: 476829) Toggle Abstract
  Photon correlations are investigated for a single laser-excited ion trapped in front of a mirror. Varying the relative distance between the ion and the mirror, photon correlation statistics can be tuned smoothly from an antibunching minimum to a bunching-like maximum. Our analysis concerns the non-Markovian regime of the ion-mirror interaction and reveals the field establishment in a half-cavity interferometer.
  (local copy)
• M. Riebe, M. Chwalla, J. Benhelm, H. Häffner, W. Hänsel, C. Roos, R. Blatt Quantum teleportation with atoms: quantum process tomography, 9 211 (2007), (ID: 499882) Toggle Abstract
  The performance of a quantum teleportation algorithm implemented on an ion trap quantum computer is investigated. First the algorithm is analysed in terms of the teleportation fidelity of six input states evenly distributed over the Bloch sphere. Furthermore, a quantum process tomography of the teleportation algorithm is carried out which provides almost complete knowledge about the algorithm.
  (local copy)
• F. Dubin, D. Rotter, M. Mukherjee, S. Gerber, R. Blatt Single-ion two-photon source, Phys. Rev. Lett. 99 183001 (2007), (ID: 535603)
• M. Chwalla, K. Kim, T. Monz, P. Schindler, M. Riebe, C. Roos, R. Blatt Precision spectroscopy with two correlated atoms, 89 483 (2007), (ID: 545098)
• M. Schulz, H. Crepaz, F. Schmidt-Kaler, J. Eschner, R. Blatt Transfer of trapped atoms between two optical tweezer potentials, J. Mod. Opt. 54 1619 (2007), (ID: 620359) Toggle Abstract
  Trapped, laser-cooled rubidium atoms are transferred between two strongly focused, horizontal, orthogonally intersecting laser beams. The transfer efficiency is studied as a function of the vertical distance between the beam axes. Optimum transfer is found when the distance equals the beam waist radius. Numerical simulations reproduce well the experimental results.

2006

Articles

• P. Bushev, D. Rotter, A. Wilson, F. Dubin, C. Becher, J. Eschner, R. Blatt, V. Steixner, P. Rabl, P. Zoller Feedback cooling of a single trapped ion, Phys. Rev. Lett. 96 043003 (2006-02-03), http://dx.doi.org/10.1103/PhysRevLett.96.043003 doi:10.1103/PhysRevLett.96.043003 (ID: 332288) Toggle Abstract
  Based on a real-time measurement of the motion of a single ion in a Paul trap, we demonstrate its electromechanical cooling below the Doppler limit by homodyne feedback control (cold damping). The feedback cooling results are well described by a model based on a quantum mechanical master equation.
• C. Roos, M. Chwalla, K. Kim, M. Riebe, R. Blatt Designer atoms for quantum metrology, Nature 443 319 (2006), http://dx.doi.org/10.1038/nature05101 doi:10.1038/nature05101 (ID: 390237) (local copy)
• J. Korbicz, O. Gühne, M. Lewenstein, H. Häffner, C. Roos, R. Blatt Generalized spin-queezing inequalities in N-qubit systems, Phys. Rev. A 74 052319 (2006), http://dx.doi.org/10.1103/PhysRevA.74.052319 doi:10.1103/PhysRevA.74.052319 (ID: 420124) Toggle Abstract
  We present detailed derivations, various improvements and application to concrete experimental data of spin squeezing inequalities formulated recently by some of us [Phys. Rev. Lett. 95, 120502 (2005)]. These inequalities generalize the concept of the spin squeezing parameter, and provide necessary and sufficient conditions for genuine 2-, or 3- qubit entanglement for symmetric states, and sufficient entanglement condition for general $N$-qubit states. We apply our method to theoretical study of Dicke states, and, in particular, to W-states of N qubits. Then, we analyze the recently experimentally generated 7- and 8-ion W-states [Nature 438, 643 (2005)]. We also present some novel details concerning this experiment. Finally, we improve criteria for detection of genuine tripartite entanglement based on entanglement witnesses.
• M. Riebe, K. Kim, P. Schindler, T. Monz, P. O. Schmidt, T. Körber, W. Hänsel, H. Häffner, C. Roos, R. Blatt Process tomography of ion trap quantum gates, Phys. Rev. Lett. 97 220407 (2006), http://dx.doi.org/10.1103/PhysRevLett.97.220407 doi:10.1103/PhysRevLett.97.220407 (ID: 423123) (local copy)

Proceedings

• H. Häffner, M. Riebe, F. Schmidt-Kaler, W. Hänsel, C. Roos, M. Chwalla, J. Benhelm, T. Körber, G. Lancaster, C. Becher, D. James, R. Blatt Controlling three atomic qubits, Physical Realizations of Quantum Computing 108 (2006), (ID: 361308) Toggle Abstract
  We present a series of experiments where up to three ions held in a Paul trap are entangled, a given number of ions is selectively read out while conditional single-quantum-bit (qubit) operations are performed coherently on the remaining ion(s). Using these techniques, we demonstrate also a state transfer of a quantum bit from one ion to another one using two measurements and entanglement between an auxiliary ion and the target ion -- also known as teleportation.
• H. Häffner, F. Schmidt-Kaler, W. Hänsel, C. Roos, P. O. Schmidt, M. Riebe, M. Chwalla, D. Chek-al-Kar, J. Benhelm, U. D. Rapol, T. Körber, C. Becher, R. Blatt Long lived entangled states, Foundations of Quantum Mechanics in the Light of New Technology ISQM-Tokyo (Tokyo, Japan, 2005-08-22) 33 (2006), (ID: 375560) Toggle Abstract
  We investigate entangled states of two trapped Calcium-40 ions. By encoding the quantum information in the Zeeman-manifold of the ground state, we observe entangled states lasting for more than 20 seconds.
• C. Roos, M. Chwalla, K. Kim, M. Riebe, R. Blatt Precision spectroscopy with entangled states: Measurement of electric quadrupole moments, Atomic physics 20 (Innsbruck, Austria, 2006-07-16) 111-118 (2006), (ID: 420335)

2005

Articles

• P. Zoller, T. Beth, D. Binosi, R. Blatt, H. J. Briegel, D. Bruss, T. Calarco, J. I. Cirac, D. Deutsch, J. Eisert, A. Ekert, C. Fabre, N. Gisin, P. Grangiere, M. Grassl, S. Haroche, A. Imamoglu, A. Karlson, J. Kempe, L. Louwenhofen, S. Kröll, G. Leuchs, M. Quantum information processing and communication, Eur. Phys. J. D 36/2 203 - 228 (2005-11-01), http://dx.doi.org/10.1140/epjd/e2005-00251-1 doi:10.1140/epjd/e2005-00251-1 (ID: 375863) Toggle Abstract
  We present an excerpt of the document “Quantum Information Processing and Communication: Strategic report on current status, visions and goals for research in Europe”, which has been recently published in electronic form at the website of FET (the Future and Emerging Technologies Unit of the Directorate General Information Society of the European Commission, http://www.cordis.lu/ist/fet/qipc-sr.htm). This document has been elaborated, following a former suggestion by FET, by a committee of QIPC scientists to provide input towards the European Commission for the preparation of the Seventh Framework Program. Besides being a document addressed to policy makers and funding agencies (both at the European and national level), the document contains a detailed scientific assessment of the state-of-the-art, main research goals, challenges, strengths, weaknesses, visions and perspectives of all the most relevant QIPC sub-fields, that we report here. Dedicated to the memory of Prof. Th. Beth, one of the pioneers of QIPC, whose contributions have had a significant scientific impact on the development as well as on the visibility of a field that he enthusiastically helped to shape since its early days.
• L. Tian, R. Blatt, P. Zoller Scalable ion trap quantum computing without moving ions, Eur. Phys. J. D 32 201-208 (2005), http://dx.doi.org/10.1140/epjd/e2004-00172-5 doi:10.1140/epjd/e2004-00172-5 (ID: 308239) Toggle Abstract
  A hybrid quantum computing scheme is studied where the hybrid qubit is made of an ion trap qubit serving as the information storage and a solid-state charge qubit serving as the quantum processor, connected by a superconducting cavity. In this paper, we extend our previous work [1] and study the decoherence, coupling and scalability of the hybrid system. We present our calculations of the decoherence of the coupled ion-charge system due to the charge fluctuations in the solid-state system and the dissipation of the superconducting cavity under laser radiation. A gate scheme that exploits rapid state flips of the charge qubit to reduce decoherence by the charge noise is designed. We also study a superconducting switch that is inserted between the cavity and the charge qubit and provides tunable coupling between the qubits. The scalability of the hybrid scheme is discussed together with several potential experimental obstacles in realizing this scheme.
• H. Häffner, W. Hänsel, C. Roos, J. Benhelm, D. Chek-al-Kar, M. Chwalla, T. Körber, U. D. Rapol, M. Riebe, P. O. Schmidt, C. Becher, O. Gühne, W. Dür, R. Blatt Scalable multi-particle entanglement of trapped ions, Nature 438 646 (2005), http://dx.doi.org/10.1038/nature04279 doi:10.1038/nature04279 (ID: 314190) Toggle Abstract
  The generation, manipulation and fundamental understanding of entanglement lies at the very heart of quantum mechanics. Entangled particles are non-interacting but are described by a common wavefunction; consequently, individual particles are not independent of each other and their quantum properties are inextricably interwoven. The intriguing features of entanglement become particularly evident if the particles can be individually controlled and physically separated. However, both the experimental realization and characterization of entanglement become exceedingly difficult for systems with many particles. The main difficulty is to manipulate and detect the quantum state of individual particles as well as to control the interaction between them. So far, entanglement of four ions or five photons has been demonstrated experimentally. The creation of scalable multiparticle entanglement demands a non-exponential scaling of resources with particle number. Among the various kinds of entangled states, the 'W state' plays an important role as its entanglement is maximally persistent and robust even under particle loss. Such states are central as a resource in quantum information processing and multiparty quantum communication. Here we report the scalable and deterministic generation of four-, five-, six-, seven- and eight-particle entangled states of the W type with trapped ions. We obtain the maximum possible information on these states by performing full characterization via state tomography, using individual control and detection of the ions. A detailed analysis proves that the entanglement is genuine. The availability of such multiparticle entangled states, together with full information in the form of their density matrices, creates a test-bed for theoretical studies of multiparticle entanglement. Independently, 'Greenberger–Horne–Zeilinger' entangled states with up to six ions have been created and analysed in Boulder.
• H. Häffner, F. Schmidt-Kaler, W. Hänsel, C. Roos, T. Körber, M. Chwalla, J. Benhelm, U. D. Rapol, C. Becher, R. Blatt Robust Entanglement, 81 151 (2005), (ID: 314806) Toggle Abstract
  It is common belief among physicists that entangled states of quantum systems lose their coherence rather quickly. The reason is that any interaction with the environment which distinguishes between the entangled sub-systems collapses the quantum state. Here we investigate entangled states of two trapped Ca+ ions and observe robust entanglement lasting for more than 20 s.
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• A. Kreuter, C. Becher, G. Lancaster, A. B. Mundt, C. Russo, H. Häffner, C. Roos, W. Hänsel, F. Schmidt-Kaler, R. Blatt, M. Safronova New experimental and theoretical approach to the 3d ²D-level lifetimes of ⁴⁰Ca⁺, Phys. Rev. A 71 032504 (2005), (ID: 314807) (local copy)
• R. Blatt Ionen in Reih und Glied, Physik Journal 4, Nr. 11 37 (2005), (ID: 327389) Toggle Abstract
  Ist eine Münze gezinkt oder nicht, d. h. weist sie Kopf und Zahl auf oder stimmen beide Seiten überein? Ein einfacher Quantenalgorithmus erlaubt es, diese Frage mit nur einem Blick auf die Münze statt zweien zu beantworten. Der „Rechner“, auf dem dieser Algorithmus ausgeführt wird, besteht nicht aus Transistoren, sondern aus kalten, eingesperrten Ionen.
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Proceedings

• H. Häffner, M. Riebe, F. Schmidt-Kaler, W. Hänsel, C. Roos, M. Chwalla, J. Benhelm, T. Körber, C. Becher, D. James, R. Blatt Teleportation with Atoms, Atomic Physics (Rio De Janeiro) 19 341-349 (2005), (ID: 314808) Toggle Abstract
  The teleportation of an atomic state accomplishes the complete transfer of information from one particle to another, employing the non-local properties of quantum mechanics. Recently, two groups have achieved the deterministic teleportation of a quantum state between a pair of trapped ions. Following closely the original proposal of Bennett et al.[1], a highly entangled pair of ions is created, a complete Bell-state projective measurement involving the source ion and one of the entangled pair is carried out, and state reconstruction conditioned on this measurement is performed on the other half of the entangled pair.
• C. Becher, J. Benhelm, D. Chek-al-Kar, M. Chwalla, W. Dür, O. Gühne, H. Häffner, W. Hänsel, T. Körber, A. Kreuter, G. Lancaster, T. Monz, E. Phillips, U. D. Rapol, M. Riebe, C. Roos, C. Russo, F. Schmidt-Kaler, R. Blatt Entanglement of trapped ions, Laser Spectroscopy ICOLS XVII 392 (2005), (ID: 327393) Toggle Abstract
  Entanglement, its generation, manipulation, measurement and fundamental understanding is at the very heart of quantum mechanics. We here report on the creation and characterization of entangled states of up to 8 trapped ions, the investigation of long-lived two-ion Bell-states and on experiments towards entangling ions and photons.

2004

Articles

• L. Tian, P. Rabl, R. Blatt, P. Zoller Interfacing Quantum-Optical and Solid-State Qubits, Phys. Rev. Lett. 92 247902 (2004), http://dx.doi.org/10.1103/PhysRevLett.92.247902 doi:10.1103/PhysRevLett.92.247902 (ID: 314633) Toggle Abstract
  We present a generic model of coupling quantum-optical and solid-state qubits, and the corresponding transfer protocols. The example discussed is a trapped ion coupled to a charge qubit (e.g., Cooper pair box). To enhance the coupling and to achieve compatibility between the different experimental setups we introduce a superconducting cavity as the connecting element.
• C. Roos, M. Riebe, H. Häffner, W. Hänsel, J. Benhelm, G. Lancaster, C. Becher, F. Schmidt-Kaler, R. Blatt Control and Measurment of Three-Qubit Entangled States, Science 304 1478 (2004), (ID: 342102) Toggle Abstract
  We report the deterministic creation of maximally entangled three-qubit states—specifically the Greenberger-Horne-Zeilinger (GHZ ) state and the W state—with a trapped-ion quantum computer.We read out one of the qubits selectively and show how GHZ andWstates are affected by this local measurement.Additionally, we demonstrate conditional operations controlled by the results from reading out one qubit.Tripartite entanglement is deterministically transformed into bipartite entanglement by local operations only.These operations are the measurement of one qubit of a GHZ state in a rotated basis and, conditioned on this measurement result, the application of single-qubit rotations.
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• C. Roos, G. Lancaster, M. Riebe, H. Häffner, W. Hänsel, S. Gulde, C. Becher, J. Eschner, F. Schmidt-Kaler, R. Blatt Bell States of Atoms with Ultralong Lifetimes and Their Tomographic State Analysis, Phys. Rev. Lett. 92 220402 (2004), (ID: 342103) Toggle Abstract
  Arbitrary atomic Bell states with two trapped ions are generated in a deterministic and preprogrammed way. The resulting entanglement is quantitatively analyzed using various measures of entanglement. For this, we reconstruct the density matrix using single qubit rotations and subsequent measurements with near-unity detection efficiency. This procedure represents the basic building block for future process tomography of quantum computations. As a first application, the temporal decay of entanglement is investigated in detail.We observe ultralong lifetimes for the Bell states, close to the fundamental limit set by the spontaneous emission from the metastable upper qubit level and longer than all reported values by 3 orders of magnitude.
• S. Gulde, M. Riebe, G. Lancaster, C. Becher, J. Eschner, H. Häffner, F. Schmidt-Kaler, R. Blatt Quantized AC-Stark shifts and their use for multiparticle entanglement and quantum gates, 65 587 (2004), (ID: 342104)
• A. Kreuter, C. Becher, G. Lancaster, A. B. Mundt, C. Russo, H. Häffner, C. Roos, J. Eschner, F. Schmidt-Kaler, R. Blatt Spontaneous Emission Lifetime of a Single Trapped Ca+ Ion in a High Finesse Cavity, Phys. Rev. Lett. 92 203002 (2004), (ID: 342120)
• C. Maurer, C. Becher, C. Russo, J. Eschner, R. Blatt A single-photon source based on a single Ca+ ion, New J. Phys. 6 94 (2004), (ID: 343914) Toggle Abstract
  We propose a deterministic source of single photons based on the vacuum-stimulated Raman transition of a single Ca+ ion trapped inside a high finesse cavity. Assuming realistic experimental parameters, the efficiency of photon emission into the cavity mode reaches 95%.
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• P. Bushev, M. A. Wilson, J. Eschner, C. Raab, F. Schmidt-Kaler, C. Becher, R. Blatt Forces between a Single Atom and Its Distant Mirror Image, Phys. Rev. Lett. 92 223602 (2004), (ID: 343934) Toggle Abstract
  An excited-state atom whose emitted light is backreflected by a distant mirror can experience trapping forces, because the presence of the mirror modifies both the electromagnetic vacuum field and the atom’s own radiation reaction field.We demonstrate this mechanical action using a single trapped barium ion. We observe the trapping conditions to be notably altered when the distant mirror is translated across an optical wavelength. The well-localized barium ion enables the spatial dependence of the forces to be measured explicitly. The experiment has implications for quantum information processing and may be regarded as the most elementary optical tweezers.
• R. Blatt, H. Häffner, C. Roos, C. Becher, F. Schmidt-Kaler Ion Trap Quantum Computing with Ca+ Ions, Quant. Inf. Proc. 3 1-5 (2004), (ID: 513993)
• M. Riebe, H. Häffner, C. Roos, W. Hänsel, J. Benhelm, G. Lancaster, T. Körber, C. Becher, F. Schmidt-Kaler, D. James, R. Blatt Deterministic quantum teleportation with atoms, Nature 429 734 (2004), (ID: 520666) Toggle Abstract
  Teleportation of a quantum state encompasses the complete transfer of information from one particle to another. The complete specification of the quantum state of a system generally requires an infinite amount of information, even for simple twolevel systems (qubits). Moreover, the principles of quantum mechanics dictate that any measurement on a system immediately alters its state, while yielding at most one bit of information. The transfer of a state from one system to another (by performing measurements on the first and operations on the second) might therefore appear impossible. However, it has been shown1 that the entangling properties of quantum mechanics, in combination with classical communication, allow quantum-state teleportation to be performed. Teleportation using pairs of entangled photons has been demonstrated2–6, but such techniques are probabilistic, requiring post-selection of measured photons. Here, we report deterministic quantum-state teleportation between a pair of trapped calcium ions. Following closely the original proposal1, we create a highly entangled pair of ions and perform a complete Bell-state measurement involving one ion from this pair and a third source ion. State reconstruction conditioned on this measurement is then performed on the other half of the entangled pair. The measured fidelity is 75%, demonstrating unequivocally the quantum nature of the process.
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2003

Articles

• F. Schmidt-Kaler, H. Häffner, S. Gulde, M. Riebe, G. Lancaster, T. Deuschle, C. Becher, W. Hänsel, J. Eschner, C. Roos, R. Blatt How to realize a universal quantum gate with trapped ions, Appl. Phys. B Las. Opt. 77 789 (2003), http://dx.doi.org/10.1007/s00340-003-1346-9 doi:10.1007/s00340-003-1346-9 (ID: 342105) Toggle Abstract
  We report the realization of an elementary quantum processor based on a linear crystal of trapped ions. Each ion serves as a quantum bit (qubit) to store the quantum information in long lived electronic states.We present the realization of single-qubit and of universal two-qubit logic gates. The twoqubit operation relies on the coupling of the ions through their collective quantized motion. A detailed description of the setup and the methods is included.
• G. Lancaster, H. Häffner, M. A. Wilson, C. Becher, J. Eschner, F. Schmidt-Kaler, R. Blatt Doppler cooling a single Ca+ ion with a violet extended-cavity diode laser, Appl. Phys. B Las. Opt. 76 805 (2003), (ID: 342107) Toggle Abstract
  We present a scheme for employing a violet extended-cavity diode laser in experiments with single, trapped ions. For this the grating-stabilised laser is spatially and spectrally filtered and referenced to a Fabry–P´erot cavity. We measure an upper limit to the line width by observing a 305-kHz FWHM beat note with the second harmonic of a titanium sapphire laser. The laser is subsequently used to optically cool a single 40Ca+ ion close to the Doppler limit.
• H. Häffner, S. Gulde, M. Riebe, G. Lancaster, C. Becher, J. Eschner, F. Schmidt-Kaler, R. Blatt Precision measurement and compensation of optical Stark shifts for an ion-trap quantum processor, Phys. Rev. Lett. 90 143602 (2003), (ID: 342109)
• F. Schmidt-Kaler, H. Häffner, M. Riebe, S. Gulde, G. Lancaster, T. Deuschle, C. Becher, C. Roos, J. Eschner, R. Blatt Realization of the Cirac-Zoller controlled-NOT quantum gate, Nature 422 408 (2003), (ID: 342111) Toggle Abstract
  Quantum computers have the potential to perform certain computational tasks more efficiently than their classical counterparts. The Cirac–Zoller proposal1 for a scalable quantum computer is based on a string of trapped ions whose electronic states represent the quantum bits of information (or qubits). In this scheme, quantum logical gates involving any subset of ions are realized by coupling the ions through their collective quantized motion. The main experimental step towards realizing the scheme is to implement the controlled-NOT (CNOT) gate operation between two individual ions. The CNOT quantum logical gate corresponds to the XOR gate operation of classical logic that flips the state of a target bit conditioned on the state of a control bit. Here we implement a CNOT quantum gate according to the Cirac–Zoller proposal1. In our experiment, two 40Ca1 ions are held in a linear Paul trap and are individually addressed using focused laser beams2; the qubits3 are represented by superpositions of two long-lived electronic states. Our work relies onrecently developed precise control of atomic phases4 and the application of composite pulse sequences adapted from nuclear magnetic resonance techniques5,6.
• S. Gulde, M. Riebe, G. Lancaster, C. Becher, J. Eschner, H. Häffner, F. Schmidt-Kaler, I. L. Chuang, R. Blatt Implementing the Deutsch-Jozsa algorithm on an ion-trap quantum computer, Nature 421 48-50 (2003), (ID: 342116) Toggle Abstract
  Determining classically whether a coin is fair (head on one side, tail on the other) or fake (heads or tails on both sides) requires an examination of each side. However, the analogous quantum procedure (the Deutsch–Jozsa algorithm1,2) requires just one examination step. The Deutsch–Jozsa algorithm has been realized experimentally using bulk nuclear magnetic resonance techniques3,4, employing nuclear spins as quantum bits (qubits). In contrast, the ion trap processor utilises5 motional and electronic quantum states of individual atoms as qubits, and in principle is easier to scale to many qubits. Experimental advances in the latter area include the realization of a two-qubit quantum gate6, the entanglement of four ions7, quantum state engineering8 and entanglement-enhanced phase estimation9. Here we exploit techniques10,11 developed for nuclear magnetic resonance to implement the Deutsch–Jozsa algorithm on an ion-trap quantum processor, using as qubits the electronic and motional states of a single calcium ion. Our ion-based implementation of a full quantum algorithm serves to demonstrate experimental procedures with the quality and precision required for complex computations, confirming the potential of trapped ions for quantum computation.
• F. Schmidt-Kaler, S. Gulde, M. Riebe, T. Deuschle, A. Kreuter, G. Lancaster, C. Becher, J. Eschner, H. Häffner, R. Blatt Coherence of qubits based on single Ca+ ions, J. Phys. B: At. Mol. Opt. Phys. 36 623-636 (2003), (ID: 342123)
• M. A. Wilson, P. Bushev, J. Eschner, F. Schmidt-Kaler, C. Becher, R. Blatt, U. Dorner Vacuum-field level shifts in a single trapped ion mediated by a single distant mirror, Phys. Rev. Lett. 91 213602 (2003), (ID: 343935) Toggle Abstract
  A distant mirror leads to a vacuum-induced level shift in a laser-excited atom. This effect has been measured with a single mirror 25 cm away from a single, trapped barium ion. This dispersive action is the counterpart to the mirror’s dissipative effect, which has been shown earlier to effect a change in the ion’s spontaneous decay [J. Eschner et al., Nature (London) 413, 495 (2001)]. The experimental data are well described by eight-level optical Bloch equations which are amended to take into account the presence of the mirror according to the model in U. Dorner and P. Zoller, Phys. Rev. A 66, 023816 (2002). Observed deviations from simple dispersive behavior are attributed to multilevel effects.
• J. Eschner, G. Morigi, F. Schmidt-Kaler, R. Blatt Laser cooling of trapped ions, J. Opt. Soc. Am. B 20 1003 (2003), (ID: 343940) Toggle Abstract
  Trapped and laser-cooled ions are increasingly used for a variety of modern high-precision experiments, for frequency standard applications, and for quantum information processing. Therefore laser cooling of trapped ions is reviewed, the current state of the art is reported, and several new cooling techniques are outlined. The principles of ion trapping and the basic concepts of laser cooling for trapped atoms are introduced. The underlying physical mechanisms are presented, and basic experiments are briefly sketched. Particular attention is paid to recent progress by elucidating several milestone experiments. In addition, a number of special cooling techniques pertaining to trapped ions are reviewed; open questions and future research lines are indicated.
• D. Leibfried, R. Blatt, C. Monroe, D. Wineland Quantum dynamics of single trapped ions, Rev. Mod. Phys. 75 281 (2003), (ID: 343943) Toggle Abstract
  Single trapped ions represent elementary quantum systems that are well isolated from the environment. They can be brought nearly to rest by laser cooling, and both their internal electronic states and external motion can be coupled to and manipulated by light fields. This makes them ideally suited for quantum-optical and quantum-dynamical studies under well-controlled conditions. Theoretical and experimental work on these topics is reviewed in the paper, with a focus on ions trapped in radio-frequency (Paul) traps.
• A. B. Mundt, A. Kreuter, C. Russo, C. Becher, D. Leibfried, J. Eschner, F. Schmidt-Kaler, R. Blatt Coherent coupling of a single Ca+ ion to a high-finesse optical cavity, Appl. Phys. B Las. Opt. 76 117-124 (2003), (ID: 343950) Toggle Abstract
  We demonstrate coherent coupling of the quadrupole S1/2 ↔ D5/2 optical transition of a single trapped 40Ca+ ion to the standing wave field of a high-finesse cavity. The dependence of the coupling on temporal dynamics and spatial variations of the intracavity field is investigated in detail. By precisely controlling the position of the ion in the cavity standing wave field and by selectively exciting vibrational statechanging transitions the ion’s quantized vibration in the trap is deterministically coupled to the cavity mode. We confirm coherent interaction of ion and cavity field by exciting Rabi oscillations with short resonant laser pulses injected into the cavity, which is frequency-stabilized to the atomic transition.
• S. Gulde, H. Häffner, M. Riebe, G. Lancaster, C. Becher, J. Eschner, F. Schmidt-Kaler, I. L. Chuang, R. Blatt Quantum information Processing with Trapped Ca+ ions, Phil. Trans. R. Soc. Lond. A 361 1-12 (2003), (ID: 619585)
• J. Eschner, C. Raab, A. B. Mundt, A. Kreuter, C. Becher, F. Schmidt-Kaler, R. Blatt Single trapped ions interacting with low- und high-finesse optical cavities, Fortschr. Phys. 51 359-368 (2003), (ID: 619586)

Proceeding

• S. Gulde, H. Häffner, M. Riebe, G. Lancaster, A. B. Mundt, A. Kreuter, C. Russo, C. Becher, J. Eschner, F. Schmidt-Kaler, I. L. Chuang, R. Blatt Quantum information processing and cavity QED experiments with trapped Ca+ ions, Atomic Physics 18 253-302 (2003), (ID: 628427)

2002

Articles

• A. B. Mundt, A. Kreuter, C. Becher, D. Leibfried, J. Eschner, F. Schmidt-Kaler, R. Blatt Coupling a single atomic quantum bit to a high finesse optical cavity, Phys. Rev. Lett. 89 103001 (2002), (ID: 343952) Toggle Abstract
  The quadrupole S1=2–D5=2 optical transition of a single trapped Ca ion, well suited for encoding a quantum bit of information, is coherently coupled to the standing wave field of a high finesse cavity. The coupling is verified by observing the ion’s response to both spatial and temporal variations of the intracavity field. We also achieve deterministic coupling of the cavity mode to the ion’s vibrational state by selectively exciting vibrational state-changing transitions and by controlling the position of the ion in the standing wave field with nanometer precision.
• H. Rohde, J. Eschner, F. Schmidt-Kaler, R. Blatt Optical decay from a Fabry-Perot cavity faster than the decay time, J. Opt. Soc. Am. B 19 1425-1429 (2002), (ID: 343970) Toggle Abstract
  The dynamical response of an optical Fabry-Perot cavity is investigated experimentally. We observe oscillations in the transmitted and reflected light intensity if the frequency of the incoupled light field is rapidly changed. In addition, the decay of a cavity-stored light field is accelerated if the phase and intensity of the incoupled light are switched in an appropriate way. The theoretical model by M. J. Lawrence et al, JOSA B 16, 523 (1999) agrees with our observations.

Proceedings

• F. Schmidt-Kaler, J. Eschner, R. Blatt, D. Leibfried, C. Roos, G. Morigi Laser Cooling of trapped ions, Laser Physics at the limit 243-260 (2002), (ID: 628430)
• C. Roos, D. Leibfried, F. Schmidt-Kaler, J. Eschner, R. Blatt Sideband cooling and state manipulation: Application in precision spectroscopy, Proc. 6th Symp. Frequency Standards and Metrology 376-384 (2002), (ID: 628432)
• F. Schmidt-Kaler, D. Leibfried, J. Eschner, R. Blatt Towards Quantum Computation with trapped Ca+ Ions, Proc. of the International school of Physics Enrico Fermi, Course CXLVIII "Experimental Quantum Computation and Information" 197-211 (2002), (ID: 628433)

2001

Articles

• J. Eschner, C. Raab, F. Schmidt-Kaler, R. Blatt Light interference from single atoms and their mirror images, Nature 413 495-498 (2001), (ID: 343953) Toggle Abstract
  A single atom emitting single photons is a fundamental source of light. But the characteristics of this light depend strongly on the environment of the atom1,2. For example, if an atom is placed between two mirrors, both the total rate and the spectral composition of the spontaneous emission can be modi®ed. Such effects have been observed using various systems: molecules deposited on mirrors3, dye molecules in an optical cavity4, an atom beam traversing a two-mirror optical resonator5±8, single atoms traversing a microwave cavity9±11 and a single trapped electron12. A related and equally fundamental phenomenon is the optical interaction between two atoms of the same kind when their separation is comparable to their emission wavelength. In this situation, light emitted by one atom may be reabsorbed by the other, leading to cooperative processes in the emission13,14. Here we observe these phenomena with high visibility by using one or two single atom(s), a collimating lens and a mirror, and by recording the individual photons scattered by the atom(s). Our experiments highlight the intimate connection between one-atom and two-atom effects, and allow their continuous observation using the same apparatus.
• S. Gulde, D. Rotter, P. Barton, F. Schmidt-Kaler, R. Blatt, W. Hogervorst Simple and efficient photoionization loading of ions for precision ion-trapping experiments, Appl. Phys. B Las. Opt. 73 861-863 (2001), (ID: 343955) Toggle Abstract
  We report a simple and efficient method to load a Paul trap with Ca+ ions. A beam of neutral atomic calcium is ionized in a two-step photo-ionization process using uv-diode lasers near 423 nm and 390 nm. Photo-ionization of a calcium beam for loading a Paul trap has first been demonstrated by Kjærgaard et al. The advantages of ourmethod are the use of cheap and easily handled diode-laser systems and the large cross section for field ionization when exciting high-lying Rydberg states. Finally, we discuss the advantages of photo-ionization for ion generation compared to loading by electron bombardment
• R. Blatt Delicate information, Nature 412 773 (2001), (ID: 343960)
• F. Schmidt-Kaler, J. Eschner, G. Morigi, C. Roos, D. Leibfried, A. B. Mundt, R. Blatt Laser cooling with electromagnetically induced transparency: Application to trapped samples of ions or neutral atoms, Appl. Phys. B Las. Opt. 73 807 (2001), (ID: 343962) Toggle Abstract
  A novel method of ground-state laser cooling of trapped atoms utilizes the absorption profile of a three- (or multi-) level system that is tailored by a quantum interference. With cooling rates comparable to conventional sideband cooling, lower final temperatures may be achieved. The method was experimentally implemented to cool a single Ca+ ion to its vibrational ground state. Since a broad band of vibrational frequencies can be cooled simultaneously, the technique will be particularly useful for the cooling of larger ion strings, thereby being of great practical importance for initializing a quantum register based on trapped ions.We also discuss its application to different level schemes and for ground-state cooling of neutral atoms trapped by a far-detuned standing wave laser field.
• H. Rohde, S. Gulde, C. Roos, P. Barton, D. Leibfried, J. Eschner, F. Schmidt-Kaler, R. Blatt Sympathetic ground state cooling and coherent manipulation with two-ion-crystals, J. Opt. B: Quantum Semiclass. Opt. 3 34 (2001), (ID: 469819)

Proceedings

• F. Schmidt-Kaler, D. Leibfried, J. Eschner, R. Blatt Quantum Computation with trapped Ca+ Ions, Proc. of the Dolomieu Winter School (2001), (ID: 628434)
• J. Eschner, C. Raab, P. Bouchev, F. Schmidt-Kaler, R. Blatt Single ions interfering with their mirror images, Laser Spectroscopy XV, Proc of the 15th International Conference (ICOLS01) 193-201 (2001), (ID: 628439)
• D. Leibfried, C. Roos, P. Barton, H. Rohde, S. Gulde, A. B. Mundt, F. Schmidt-Kaler, J. Eschner, R. Blatt Towards quantum information with trapped Calcium ions, Quantum Communication, Computing and Measurement 3 179 (2001), (ID: 628450)
• D. Leibfried, C. Roos, P. Barton, H. Rohde, S. Gulde, A. B. Mundt, G. Reymond, M. Lederbauer, F. Schmidt-Kaler, J. Eschner, R. Blatt Experiments towards quantum information with trapped Calcium ions, Atomic Physics (Proceedings of the ICAP 2000) 17 130 (2001), (ID: 628457)

2000

Articles

• C. Roos, D. Leibfried, A. B. Mundt, F. Schmidt-Kaler, J. Eschner, R. Blatt Experimental demonstration of ground state laser cooling with electromagnetically induced transparency, Phys. Rev. Lett. 85 5547 (2000), (ID: 344232) Toggle Abstract
  A laser cooling method for trapped atoms is described which achieves ground state cooling by exploiting quantum interference in a driven L-shaped arrangement of atomic levels. The scheme is technically simpler than existing methods of sideband cooling, yet it can be significantly more efficient, in particular when several motional modes are involved, and it does not impose restrictions on the transition linewidth. We study the full quantum mechanical model of the cooling process for one motional degree of freedom and show that a rate equation provides a good approximation.
• H. Rohde, S. Gulde, C. Roos, P. Barton, D. Leibfried, J. Eschner, F. Schmidt-Kaler, R. Blatt Sympathetic ground state cooling and coherent manipulation with two-ion-crystals, J. Opt. B: Quantum Semiclass. Opt. 3 S3 (2000), (ID: 344234) Toggle Abstract
  We have cooled a two-ion crystal to the ground-state of its collective modes of motion. Laser cooling, more specifically resolved sideband cooling, is performed sympathetically by illuminating only one of the two 40Ca+ ions in the crystal. The heating rates of the motional modes of the crystal in our linear trap have been measured, and we found them considerably smaller than those previously reported by Turchette et al (2000 Phys.Rev.A 61 063418) in the case of trapped 9Be+ ions. After the ground state is prepared, coherent quantum state manipulation of the atomic population can be performed. Up to 12 Rabi oscillations are observed, showing that many coherent manipulations can be achieved. Coherent excitation of each ion individually and ground state cooling are important tools for the realization of quantum information processing in ion traps.
• R. Blatt Push-button entanglement, Nature 404 231 (2000), (ID: 344235) Toggle Abstract
  Quantum mechanics allows matter to be prepared in a strangely correlated way called entanglement. In future, large numbers of entangled particles may be put to work in quantum computers and precise quantum measurements.
• A. Steane, C. Roos, D. Stevens, A. B. Mundt, D. Leibfried, F. Schmidt-Kaler, R. Blatt Speed of ion trap quantum information processors, Phys. Rev. A 62 042305 (2000), (ID: 344236) Toggle Abstract
  We investigate theoretically the speed limit of quantum gate operations for ion trap quantum information processors. The proposed methods use laser pulses for quantum gates that entangle the electronic and vibrational degrees of freedom of the trapped ions. Two of these methods are studied in detail and for both of them the speed is limited by a combination of the recoil frequency of the relevant electronic transition, and the vibrational frequency in the trap. We have experimentally studied the gate operations below and above this speed limit. In the latter case, the fidelity is reduced, in agreement with our theoretical findings.
• C. Raab, J. Eschner, J. Bolle, H. Oberst, F. Schmidt-Kaler, R. Blatt Motional sidebands and direct measurement of the cooling rate in the resonance fluorescence of a single trapped ion, Phys. Rev. Lett. 85 538 (2000), (ID: 344237) Toggle Abstract
  Resonance fluorescence of a single trapped ion is spectrally analyzed using a heterodyne technique. Motional sidebands due to the oscillation of the ion in the harmonic trap potential are observed in the fluorescence spectrum. From the width of the sidebands the cooling rate is obtained and found to be in agreement with the theoretical prediction.
• H. Nägerl, C. Roos, D. Leibfried, H. Rohde, G. Thalhammer, J. Eschner, F. Schmidt-Kaler, R. Blatt Investigating a qubit candidate: Spectroscopy on the S1/2 to D5/2 transition of a trapped calcium ion in a linear Paul trap, Phys. Rev. A 61 023405 (2000), (ID: 344239) Toggle Abstract
  A single 40Ca1 ion is confined in a linear Paul trap and Doppler-cooled on the S1/2 to P1/2 dipole transition. Then the narrow quadrupole S1/2 to D5/2 transition at 729 nm is probed. The observed spectrum is interpreted in terms of the Zeeman substructure superimposed with oscillation sidebands due to the harmonic motion in the trap. The height of the motional sidebands provides a sensitive method to determine the ion’s temperature and thus allows us to test sub-Doppler laser cooling schemes needed for quantum state preparation and quantum computation. We also observe the dynamics induced by Rabi oscillations on a carrier transition and interpret it in terms of the thermal state which is reached after Doppler cooling.
• F. Schmidt-Kaler, C. Roos, H. Nägerl, H. Rohde, S. Gulde, A. B. Mundt, M. Lederbauer, G. Thalhammer, T. Zeiger, P. Barton, L. Hornekaer, G. Reymond, D. Leibfried, J. Eschner, R. Blatt Ground state cooling, quantum state engineering, and study of decoherence in Paul traps, J. Mod. Opt. 47 2573 (2000), (ID: 469818)
• H. Nägerl, C. Roos, H. Rohde, D. Leibfried, J. Eschner, F. Schmidt-Kaler, R. Blatt Addressing and cooling of single ions in Paul traps, Fortschr. Phys. 48 623 (2000), (ID: 619587)

Proceedings

• F. Schmidt-Kaler, H. Nägerl, D. Leibfried, R. Blatt, M. Brune, J. M. Raimond, S. Haroche Cavity QED-Experiments for Quantum Information: Atoms in Cavities and Trapped Ions, The Physics of Quantum Information 133-162 (2000), (ID: 628464)
• H. Nägerl, F. Schmidt-Kaler, J. Eschner, R. Blatt, W. Lange, H. Baldauf, H. Walther Linear Ion Traps for Quantum Computation, The Physics of Quantum Information 163-176 (2000), (ID: 628465)

1999

Articles

• C. Roos, T. Zeiger, H. Rohde, H. Nägerl, J. Eschner, D. Leibfried, F. Schmidt-Kaler, R. Blatt Quantum state engineering on an optical transition and decoherence in a Paul trap, Phys. Rev. Lett. 83 4713 (1999), (ID: 344238)
• H. Nägerl, D. Leibfried, H. Rohde, G. Thalhammer, J. Eschner, F. Schmidt-Kaler, R. Blatt Laser addressing of individual ions in a linear ion trap, Phys. Rev. A 60 145 (1999), (ID: 344241) Toggle Abstract
  Single ions in a linear string have been addressed with a tightly focused laser beam and an acousto-optic deflector. The excitation into a long-lived metastable level is detected with a quantum jump technique. Singlequantum bit operations for quantum information processing with trapped ions are shown to be feasible.
• C. Raab, J. Boll, H. Oberst, J. Eschner, F. Schmidt-Kaler, R. Blatt Diode laser spectrometer at 493 nm for single trapped Ba+ ions (erratum), 69 253 (1999), (ID: 635069)

Proceedings

• C. Raab, J. Bolle, H. Oberst, J. Eschner, F. Schmidt-Kaler, D. Leibfried, R. Blatt Motional effects in the fluorescence spectrum of a single Ba+ ion, Laser Spectroscopy XIV 346 (1999), (ID: 628459)
• F. Schmidt-Kaler, C. Roos, H. Nägerl, H. Rohde, S. Gulde, A. B. Mundt, T. Zeiger, G. Reymond, G. Thalhammer, D. Leibfried, J. Eschner, R. Blatt Single ions in Paul traps, Laser Spectroscopy XIV 246 (1999), (ID: 628460)
• H. Nägerl, C. Roos, H. Rohde, D. Leibfried, J. Eschner, F. Schmidt-Kaler, R. Blatt Single Ions in Paul traps, Decoherence: Theoretical, Experimental and Conceptual Problems (1999), (ID: 628462)

1998

Articles

• H. Nägerl, D. Leibfried, F. Schmidt-Kaler, J. Eschner, R. Blatt Coherent excitation of normal modes in a string of Ca+ ions, Opt. Express 3 89 (1998), (ID: 344244) Toggle Abstract
  Crystal structures of Calcium ions have been prepared in a linear Paul trap and their collective motion excited with resonant rf-fields. The trapped ions are laser-cooled and images of the fluorescing ions are obtained with a CCD camera and show high spatial resolution. Crystals with up to 15 ions arrange in a linear string and their eigenmodes can subsequently be selectively excited. The collective motion of the string can then be observed via the CCD images.ons arrange in a linear string and their eigen-modes can subsequently be selectively excited. The collective motion of the string can then be observed via the CCD images.
• C. Raab, J. Bolle, H. Oberst, J. Eschner, F. Schmidt-Kaler, R. Blatt Diode laser spectrometer at 493 nm for single trapped Ba+ ions, Appl. Phys. B Las. Opt. 67 683 (1998), (ID: 344245) Toggle Abstract
  A diode laser spectrometer at 493 nm is described, which is especially suited for spectroscopy of single trapped BaC ions. Frequency doubling of a 100mW diode laser at 986 nm results in up to 60mW output power at 493 nm in a bandwidth of less than 60 kHz with respect to the cavity used for locking. Reference frequencies of 18 spectral lines of Te2 near the 493 nm resonance of BaC have been measured using modulation transfer spectroscopy. The fluorescence excitation spectrum of a single BaC ion, measured with this laser, exhibits well-resolved dark resonances, which confirms the versatility of the system for quantumoptical experiments.
• H. Nägerl, W. Bechter, J. Eschner, F. Schmidt-Kaler, R. Blatt Ion strings for quantum gates, Appl. Phys. B Las. Opt. 66 603 (1998), (ID: 344246) Toggle Abstract
  Crystal structures of calcium ions have been prepared in a linear Paul trap. The trapped ions are laser-cooled by simultaneous resonant excitation near 397 nm and 866 nm. Images of the fluorescing ions are obtained with a CCD camera and show the individual ions spatially resolved. Complex crystal structures of more than 60 ions have been observed whereas smaller crystals with up to 10 ions arrange in a linear string. Measured distances between the ions in strings of different lengths are in good agreement with expected values obtained from a harmonic trap potential. The application of a calcium ion string for quantum computation is discussed.

Proceeding

• H. Nägerl, W. Bechter, J. Eschner, F. Schmidt-Kaler, R. Blatt Trapped Ions for Quantum Gates and Cavity QED, Laser Spectroscopy XIII 183 (1998), (ID: 628466)

1996

Articles

• R. Blatt Quantum tricks in ion traps, Physics World 9 (6) 25 (1996), (ID: 344247)
• R. Blatt, P. Zoller Quantengatter für Quantenrechner, Physikalische Blätter 52 205 (1996), (ID: 344248)
• J. F. Poyatos, J. I. Cirac, R. Blatt, P. Zoller Trapped ions in the strong excitation regime: ion interferometry and non-classical states, Phys. Rev. A 54 1532 (1996), (ID: 344249)
• J. F. Poyatos, R. Walser, J. I. Cirac, R. Blatt, P. Zoller Motion tomography of a single trapped ion, Phys. Rev. A 53 R1966 (1996), (ID: 344250)
• D. Reiss, A. Lindner, R. Blatt Cooling of trapped multilevel atoms: A numerical analysis, Phys. Rev. A 54 5133 (1996), (ID: 619590)
• J. I. Cirac, A. S. Parkins, R. Blatt, P. Zoller Non-Classical States of Motion in Ion Traps, Adv. Atom. Molec. and Opt. Physics 37 238-296 (1996), (ID: 619592)

Proceeding

• R. Blatt, J. F. Poyatos, J. I. Cirac, P. Zoller Quantum motion of a trapped ion: Schrödinger cats, ion interferometry and quantum tomography, Proceedings of the 5th Symposium on Frequency Standards and Metrology 20 (1996), (ID: 629057)

1995

Articles

• R. Blatt, J. I. Cirac, P. Zoller Trapping states of motion with cold ions, Phys. Rev. A 52 518–524 (1995), http://dx.doi.org/10.1103/PhysRevA.52.518 doi:10.1103/PhysRevA.52.518 (ID: 305623) Toggle Abstract
  We describe a simple technique to prepare Fock states of the motion of an atom trapped in a harmonic potential. The method is based on using two lasers which interact with a weak and strong transition of the atom successively. Application of a well-defined pulse length on the weak transition leaves the population of some states out of the oscillator manifold unchanged (trapping states). Interaction on the strong transition with a second laser is used to repopulate one of these states via a third intermediate level.
• R. Blatt, J. I. Cirac, A. S. Parkins, P. Zoller Quantum motion of trapped ions, Physica Scripta T59 294 (1995), (ID: 305661)
• M. Schubert, I. Siemers, R. Blatt, W. Neuhauser, P. E. Toschek Transient internal dynamics of a multi-level ion, Phys. Rev. A 52 2994 (1995), (ID: 619595)
• B. Appasamy, I. Siemers, Y. Stalgies, J. Eschner, R. Blatt, W. Neuhauser, P. E. Toschek Quantized infrared-optical triple resonance on a single cold Barium ion, 60 473 (1995), (ID: 619596)

Proceeding

• R. Blatt Spectroscopy and Quantum Optics with Stored Ions, Atomic Physics 14 219 (1995), (ID: 628468)

1994

Articles

• J. I. Cirac, R. Blatt, P. Zoller Nonclassical states of motion in a three-dimensional ion trap by adiabatic passage, Phys. Rev. A 49 R3174–R3177 (1994-05-05), http://dx.doi.org/10.1103/PhysRevA.49.R3174 doi:10.1103/PhysRevA.49.R3174 (ID: 375186) Toggle Abstract
  A scheme for the preparation of nonclassical states of motion in a three-dimensional harmonic ion trap is proposed. The technique is based on adiabatic passage along dressed energy levels of the strongly coupled ion-trap system by varying the laser frequency. ©1994 The American Physical Society
• I. Marzoli, J. I. Cirac, R. Blatt, P. Zoller Laser cooling of trapped three-level ions: Designing two-level systems for sideband cooling, Phys. Rev. A 49 2771–2779 (1994-04-04), http://dx.doi.org/10.1103/PhysRevA.49.2771 doi:10.1103/PhysRevA.49.2771 (ID: 375175) Toggle Abstract
  Laser cooling of three-level ions of the cascade, vee, and lambda configuration is considered with simultaneous excitation on a weak and strong transition. Choosing appropriate parameters for the Rabi frequencies and the respective detunings allows one to eliminate adiabatically one of the levels. The thus designed two-level ion can acquire decay rates Γ’ smaller than the trap frequency ν such that sideband cooling becomes possible and accordingly allows cooling of the ion to its lowest oscillator state 〈n〉=0 in the trap. Explicit expressions for the detunings and the Rabi frequencies for sideband cooling of the effective two-level system are derived. ©1994 The American Physical Society
• J. I. Cirac, R. Blatt, A. S. Parkins, P. Zoller Quantum collapse and revival in the motion of a single trapped ion, Phys. Rev. A 49 1202–1207 (1994-02-02), http://dx.doi.org/10.1103/PhysRevA.49.1202 doi:10.1103/PhysRevA.49.1202 (ID: 375187) Toggle Abstract
  Within certain limits, the dynamics of a single trapped ion oscillating about the node of a standing-wave light field is described by the Jaynes-Cummings model, which is routinely used for cavity-QED experiments. We propose a technique to measure quantum collapse and revival in the population inversion of a single trapped ion, and show that this method is uniquely suited for the measurement of final temperatures of the trapped ion as well as for the analysis of nonclassical states of the ion motion, such as Fock and squeezed states. The results are discussed with particular consideration given to the effects of a finite laser bandwidth. ©1994 The American Physical Society
• J. I. Cirac, L. J. Garay, R. Blatt, A. S. Parkins, P. Zoller Laser cooling of trapped ions: The influence of micromotion, Phys. Rev. A 49 421–432 (1994-01-01), http://dx.doi.org/10.1103/PhysRevA.49.421 doi:10.1103/PhysRevA.49.421 (ID: 375185) Toggle Abstract
  Laser cooling of a single trapped ion in a Paul trap is discussed theoretically in the Lamb-Dicke limit, with full consideration of the time dependence of the trapping potential. Resulting mean kinetic energies are defined as time averages over one period of the micromotion and are compared with final temperatures expected from the laser-cooling treatment with harmonic traps. For laser-atom detunings close to the micromotion frequency the results differ significantly from those expected for a harmonic trap potential. A physical interpretation is given and simple formulas are derived for the strong confinement case. ©1994 The American Physical Society
• P. Courteille, L. S. Ma, W. Neuhauser, R. Blatt Frequency Measurement in 130Te2 near 467 nm, 59 187 (1994), (ID: 619597)
• J. Walz, I. Siemers, M. Schubert, W. Neuhauser, R. Blatt, E. Teloy Ion storage in the rf-octupole trap, Phys. Rev. A 50 4122 (1994), (ID: 619694)

Proceeding

• J. I. Cirac, R. Blatt, A. S. Parkins, P. Zoller Nonclassical states of motion and quantum collapse and revival in an ion trap, Quantum Optics VI in Springer Proceedings in Physics 77 112 (1994), (ID: 628469)

1993

Articles

• J. I. Cirac, R. Blatt, A. S. Parkins, P. Zoller Spectrum of resonance fluorescence from a single trapped ion, Phys. Rev. A 48 2169–2181 (1993-09-03), http://dx.doi.org/10.1103/PhysRevA.48.2169 doi:10.1103/PhysRevA.48.2169 (ID: 375421) Toggle Abstract
  The spectrum of resonance fluorescence of a single trapped and laser-cooled ion is studied theoretically. The quantum motion of the trapped particle manifests itself in the form of narrow motional sidebands in the fluorescence spectrum. For our calculations it is assumed that the ion is confined to dimensions much smaller than the optical wavelength (Lamb-Dicke limit) and the approach is valid for multilevel systems, general trapping potentials, and for both traveling-wave and standing-wave configurations. The motional sidebands in the spectrum have asymmetric amplitudes and this asymmetry is shown to depend on the ion energy, the detector position, and the choice of standing- or traveling-wave laser excitation. ©1993 The American Physical Society
• J. I. Cirac, R. Blatt, A. S. Parkins, P. Zoller Laser cooling of trapped ions with polarization gradients, Phys. Rev. A 48 1434–1445 (1993-08-02), http://dx.doi.org/10.1103/PhysRevA.48.1434 doi:10.1103/PhysRevA.48.1434 (ID: 375420) Toggle Abstract
  Laser cooling of a single trapped ion with Zeeman substructure below the Doppler limit is considered theoretically. The laser field consists of two counterpropagating beams linearly polarized in different directions, and the internal atomic transition is Jg=1/2→Je=3/2. The ion is assumed to be localized to spatial dimensions smaller than the optical wavelength (Lamb-Dicke limit) and placed at a specific position with respect to the laser beams. Under the assumption that the rate for optical pumping between the atomic ground states defines the smallest time constant in the system, analytic expressions for the final energy and the cooling rates are derived, with both a semiclassical and a full quantum treatment. The results show that laser cooling of a trapped ion using polarization gradients leads to very low energies. These energies are insensitive to the precise localization of the ion with respect to the lasers, the angle between the direction of the polarizations of the laser beams, and the detuning of the cooling laser. ©1993 The American Physical Society
• J. I. Cirac, A. S. Parkins, R. Blatt, P. Zoller Cooling of a trapped ion coupled strongly to a quantized cavity mode, Opt. Com. 97 353-359 (1993-04-01), http://dx.doi.org/10.1016/0030-4018(93)90502-V doi:10.1016/0030-4018(93)90502-V (ID: 375419) Toggle Abstract
  The interaction of a trapped two-level ion, confined in a harmonic potential, with a quantized cavity mode of the radiation field is studied theoretically. The ion is considered to be spatially localized on the scale of the optical wavelength (Lamb-Dicke limit), and the ion-cavity-mode coupling is assumed to be larger than or comparable to the spontaneous emission and cavity-mode loss rates. With broadband thermal light driving the cavity mode, we show that the cooling rates and final temperatures of the trapped-ion motion reflect the Jaynes-Cummings energy spectrum of the strongly-coupled ion-cavity system. 1 Present address: Departamento de Fisica Aplicada, Facultad de Ciencias, Paseo Universidad 4, 13071 Ciudad Real, Spain. 2 Permanent address: I. Institut für Laserphysik, Jungiusstr. 9, W-2000 Hamburg 36, Germany.
• J. I. Cirac, R. Blatt, A. S. Parkins, P. Zoller Preparation of Fock states by observation of quantum jumps in an ion trap, Phys. Rev. Lett. 70 762–765 (1993-02-06), http://dx.doi.org/10.1103/PhysRevLett.70.762 doi:10.1103/PhysRevLett.70.762 (ID: 375422) Toggle Abstract
  We propose a technique for the preparation of Fock states of a harmonic oscillator strongly coupled to a single two-level atomic transition based on the observation of quantum jumps. Examples are taken from the fields of cavity QED and ion trapping, where photon number states and number states of the quantized atomic motion may be prepared, respectively. ©1993 The American Physical Society
• J. I. Cirac, A. S. Parkins, R. Blatt, P. Zoller ‘‘Dark’’ squeezed states of the motion of a trapped ion, Phys. Rev. Lett. 70 556–559 (1993-02-01), http://dx.doi.org/10.1103/PhysRevLett.70.556 doi:10.1103/PhysRevLett.70.556 (ID: 375418) Toggle Abstract
  We propose a scheme for preparing coherent squeezed states of motion in an ion trap based on the multichromatic excitation of a trapped ion by standing- and traveling-wave light fields. The squeezed state is produced when the beat frequency between two standing-wave light fields is equal to twice the trap frequency, and is indicated by a ‘‘dark resonance’’ in the fluorescence emitted by the ion. ©1993 The American Physical Society
• V. Enders, P. Courteille, R. Huesmann, L. S. Ma, W. Neuhauser, R. Blatt, P. E. Toschek Microwave-Optical Double Resonance on a Single Laser-Cooled 171Yb+ Ion, 24 325 (1993), (ID: 619599)
• J. Walz, I. Siemers, M. Schubert, W. Neuhauser, R. Blatt Motional Stability of a Nonlinear Parametric Oscillator: Ion Storage in the RF Octupole Trap, 21 183 (1993), (ID: 619622)
• L. S. Ma, P. Courteille, G. Ritter, W. Neuhauser, R. Blatt Modulation-Transfer Spectroscopy in Te2 at 467 nm, 57 159 (1993), (ID: 619690)

Proceedings

• J. I. Cirac, R. Blatt, A. S. Parkins, P. Zoller Non-classical states of motion in an ion trap, Fundamentals of Quantum Optics III 156 (1993), (ID: 628470)
• R. Blatt, M. Schubert, I. Siemers, W. Neuhauser, W. Vogel Correlations of the resonance fluorescence from a single trapped ion, Fundamentals of Quantum Optics III 146 (1993), (ID: 628471)

1992

Articles

• J. I. Cirac, R. Blatt, P. Zoller, W. D. Phillips Laser cooling of trapped ions in a standing wave, Phys. Rev. A 46 2668–2681 (1992-09-01), http://dx.doi.org/10.1103/PhysRevA.46.2668 doi:10.1103/PhysRevA.46.2668 (ID: 375895) Toggle Abstract
  Laser cooling of trapped ions in a standing- and running-wave configuration is discussed theoretically. The ions are assumed to be spatially localized on the scale provided by the wavelength of the laser (Lamb-Dicke limit). A master equation for the center-of-mass distribution of the ion is derived for a multilevel system, and explicit results are presented for two- and three-level systems and harmonic trapping potentials. For the two-level system located at the node of the standing wave, we find final temperatures that are a factor of 2 lower than the limit for a running wave and cooling rates that do not saturate with the laser intensity. At the point of maximum gradient of the standing wave, blue detuned cooling is found that is analogous to the Sisyphus cooling of free atoms. For a three-level system we compare our results with those of Wineland, Dalibard, and Cohen-Tannoudji [J. Opt. Soc. Am. B 9, 32 (1992)]. ©1992 The American Physical Society
• M. Schubert, I. Siemers, R. Blatt, W. Neuhauser, P. E. Toschek Photon Anti-Bunching and Non-Poissonian Fluorescence of a Single Three-Level Ion, Phys. Rev. Lett. 68 3016 (1992), (ID: 619624)
• I. Siemers, M. Schubert, R. Blatt, W. Neuhauser, P. E. Toschek The 'Trapped State' of a Trapped Ion - Line Shifts and Shape, 18 139 (1992), (ID: 619626)
• R. Blatt, P. Gill, R. Thompson Current Perspectives on The Physics of Trapped Ions, J. Mod. Opt. 39 193 (1992), (ID: 619627)
• V. Enders, P. Courteille, W. Neuhauser, R. Blatt Microwave Spectroscopy of Motional Sidebands in Clouds of Trapped Ions, J. Mod. Opt. 39 325 (1992), (ID: 619629)
• W. Vogel, R. Blatt Resonance fluorescence from three-level atoms: Atomic coherences and squeezed light, Phys. Rev. A 45 3319 (1992), (ID: 619634)
• M. Desaintfuscien, M. Schubert, I. Siemers, R. Blatt Kinetic energy and spatial width of ion clouds in Paul traps, Appl. Phys. B54 246 (1992), (ID: 628490)

Proceedings

• I. Siemers, V. Enders, R. Blatt, W. Neuhauser, P. E. Toschek Controlling an Optical Frequency by a Single Ion, Proc. 6th European Frequency and Time Forum, ESTEC, ESA SP-340 421 (1992), (ID: 628473)
• I. Siemers, M. Schubert, V. Enders, R. Blatt, W. Neuhauser, P. E. Toschek Precision Spectroscopy on Trapped Ions, Proceedings of the 10th International Conference on Laser Spectroscopy 65 (1992), (ID: 628474)

1991

Articles

• H. Harde, J. Hattendorf-Ledwoch, H. Lehmitz, R. Blatt Self-induced Zeeman coherences in trapped Yb+ ions, 53 131 (1991), (ID: 619635)
• R. Casdorff, V. Enders, R. Blatt, W. Neuhauser, P. E. Toschek A 12-GHz Standard Clock on Trapped Ytterbium Ions, Ann. Physik (Leipzig) 48 41 (1991), (ID: 619637)

1990

Article

• M. Schubert, I. Siemers, R. Blatt Kinetic energy and spatial width of ion clouds in Paul traps, Appl. Phys. B51 414 (1990), (ID: 628489)

1989

Articles

• I. Steiner, V. Enders, F. Elsner, W. Neuhauser, P. E. Toschek, R. Blatt, J. Helmcke A Dye Ring Laser Spectrometer for Precision Spectroscopy, 49 251 (1989), (ID: 619639)
• M. Schubert, I. Siemers, R. Blatt Spatially Localized Optical Pumping in Paul Traps, J. Opt. Soc. Am. B 6 2159 (1989), (ID: 619640)
• M. Schubert, I. Siemers, R. Blatt Line Shape of Three-Level Ions in Paul Traps, Phys. Rev. A 39 5098 (1989), (ID: 619641)
• H. Lehmitz, J. Hattendorf-Ledwoch, R. Blatt, H. Harde Population Trapping in Excited Yb+ Ions, Phys. Rev. Lett. 62 2108 (1989), (ID: 619642)

Proceedings

• R. Casdorff, V. Enders, R. Blatt, W. Neuhauser, P. E. Toschek Progress Towards a Frequency Standard Based on Trapped Yb+ Ions, Proc. Int. Symp. on Electromagn. Metrology (1989), (ID: 628476)
• T. Sauter, H. Gilhaus, W. Neuhauser, R. Blatt, P. E. Toschek Nonlinear Effects in Trapped Three-Level Ions, Frequency Standards and Metrology 300 (1989), (ID: 628477)
• R. Blatt, R. Casdorff, V. Enders, W. Neuhauser, P. E. Toschek New Frequency Standards Based on Yb+, Frequency Standards and Metrology 306 (1989), (ID: 628478)

1988

Articles

• R. Blatt, P. Zoller Quantum jumps in atomic systems, European Journal of Physics 9 250-256 (1988-10-04), http://dx.doi.org/10.1088/0143-0807/9/4/002 doi:10.1088/0143-0807/9/4/002 (ID: 376844) Toggle Abstract
  The authors summarise the principal aspects of the recent discussion regarding the observation of Bohr's quantum jumps in resonance fluorescence from three-level systems. The experimental results are reviewed and possible applications are given.
• H. Gilhaus, T. Sauter, W. Neuhauser, R. Blatt, P. E. Toschek On the Light Force upon a Single Trapped Three-Level Ion, 69 25 (1988), (ID: 619652)
• T. Sauter, H. Gilhaus, I. Siemers, R. Blatt, W. Neuhauser, P. E. Toschek On the Photo-Dynamics of Single Ions in a Trap, Z. Physik D 10 153 (1988), (ID: 619658)
• I. Siebers, R. Blatt, T. Sauter, W. Neuhauser Dynamics of Ion Clouds in Paul Traps, Phys. Rev. A 38 5121 (1988), (ID: 619659)
• T. Sauter, H. Gilhaus, W. Neuhauser, R. Blatt, P. E. Toschek Kinetics of a Single Trapped Ion: Multistability and Stimulated Two-Photon Light Force, 7 317 (1988), (ID: 619661)
• R. Casdorff, R. Blatt Ordered Structures and Statistical Properties of Ion Clouds in Paul Traps, 45 175 (1988), (ID: 619662)
• T. Sauter, R. Blatt, W. Neuhauser, P. E. Toschek Quantum Jumps in a Single Ion, Physica Scripta T22 128 (1988), (ID: 619663)
• I. Siemers, R. Blatt, T. Sauter, W. Neuhauser The kinetic energy of ion clouds in Paul traps, Physica Scripta T22 240 (1988), (ID: 619664)
• R. Blatt, G. Lafyatis, W. D. Phillips, S. Stenholm, D. Wineland Cooling in traps, Physica Scripta T22 216 (1988), (ID: 619666)

1987

Proceedings

• T. Sauter, W. Neuhauser, R. Blatt, P. E. Toschek Quantum Jumps and Related Phenomena of Single Ions and Small Ion Clouds, Laser Spectroscopy VIII 121 (1987), (ID: 628479)
• T. Sauter, R. Blatt, W. Neuhauser, P. E. Toschek Quantum Jumps Observed in Single-Ion Fluorescence, Fundamentals of Quantum Optics II,Lecture Notes in Physics 282 111 (1987), (ID: 628480)
• T. Sauter, R. Blatt, W. Neuhauser, P. E. Toschek 'Quantum Jumps' Observed in the Fluorescence of a Single Ion, AIP Optical Science and Engineering Series 6: Advances in Laser Science II 337 (1987), (ID: 628481)

1986

Articles

• R. Blatt, W. Ertmer, P. Zoller, J. Hall Atomic-beam cooling: A simulation approach, Phys. Rev. A 34 3022–3033 (1986-10-04), http://dx.doi.org/10.1103/PhysRevA.34.3022 doi:10.1103/PhysRevA.34.3022 (ID: 377060) Toggle Abstract
  Laser cooling af atoms in an atomic beam is studied theoretically by using a simulation approach derived from a pure-state analysis of resonant radiation pressure. Detailed numerical results are presented discussing the form of the atomic velocity distribution (and its minimum achievable width) and effects due to the spatial (focusing) and frequency variation of a Gaussian laser beam. A comparison of these results with recent experiments is given. ©1986 The American Physical Society
• R. Blatt, P. Zoller, G. Holzmüller, I. Siemers Brownian motion of a parametric oscillator: A model for ion confinement in radio frequency traps, Zeitschrift für Physik D; Atoms, Molecules and Clusters 4 121 - 126 (1986-06-01), http://dx.doi.org/10.1007/BF01437349 doi:10.1007/BF01437349 (ID: 376916) Toggle Abstract
  The distribution function for Brownian motion of a parametric oscillator is calculated exactly with the help of continued fraction expansions in the long time limit. We derive expressions for the energy and the widths of the spatial and velocity distribution. Our results are relevant to understand confinement of particles in radio frequency ion traps.
• T. Sauter, R. Blatt, W. Neuhauser, P. E. Toschek Quantum Jumps Observed in the Fluorescence of a Single Ion, 60 287 (1986), (ID: 619668)
• T. Sauter, W. Neuhauser, R. Blatt, P. E. Toschek Observation of Quantum Jumps, Phys. Rev. Lett. 57 1696 (1986), (ID: 619670)
• J. Bialas, R. Blatt, W. Neuhauser, P. E. Toschek Ultrasensitive Detection of Light Absorption by Few Ions, 59 27 (1986), (ID: 619671)

Proceeding

• J. Bialas, R. Blatt, W. Neuhauser, P. E. Toschek Cross Saturated Side-Band Absorption: The Limits of Detectivity, Proceedings of the 1. ILS-Conference, Dallas 1985 in AIP Optical Science and Engineering Series 6: Advances in Laser Science I (1986), (ID: 629367)

1985

Articles

• W. Ertmer, R. Blatt, J. Hall, M. Zhu Laser Manipulation of Atomic Beam Velocities: Demonstration of Stopped Atoms and Velocity Reversal, Phys. Rev. Lett. 54 996 (1985), (ID: 619672)
• R. Blatt, W. Ertmer, J. Hall Atomic Beam Cooling by Counterpropagating Laser Radiation, Acta Physica Austriaca 57 151 (1985), (ID: 619695)

Proceeding

• W. Ertmer, R. Blatt, J. Hall, M. Zhu "Laser Manipulation of Atomic Beam Velocities:Demonstration of Stopped Atoms and Velocity Reversal", Beitrag zum Seminar "Fundamentals of Laser Interactions", Obergurgl, Ötztal, Austria, Lecture Notes in Physics 229, 309 (1985), (ID: 629369)

1984

Articles

• W. Ertmer, R. Blatt, J. Hall Candidate Atoms and Ions for Frequency Standards Research Using Laser Radiative Cooling Techniques, Progr. Quant. Electr. 8 249 (1984), (ID: 619674)
• R. Blatt, W. Ertmer, J. Hall Cooling of an Atomic Beam with Frequency Sweep Techniques Progr., Quant. Electr. 8 237 (1984), (ID: 619675)
• W. Becker, R. Blatt, G. Werth High Resolution Microwave Spectroscopy on Trapped Ba+ Ions PMFCII, NBS Spec. Publ. 617 99 (1984), (ID: 619680)

1983

Article

• R. Blatt, H. Schnatz, G. Werth Determination of the 171Yb+ Ground-State Hyperfine Separation, Z. Phys. A 312 143 (1983), (ID: 619676)

1982

Articles

• R. Blatt, H. Schnatz, G. Werth Ultrahigh Resolution Microwave Spectroscopy on Trapped 171Yb+ Ions, Phys. Rev. Lett. 48 1601 (1982), (ID: 619679)
• R. Blatt, G. Werth Precision Determination of the Ground-State Hyperfine Splitting in 137Ba+ Using the Ion Storage Technique, Phys. Rev. A 25 1476 (1982), (ID: 619684)

1981

Articles

• R. Blatt, G. Werth Precision Ground State Hfs Separation of 137Ba+, Zeitschrift für Physik A 299 93 (1981), (ID: 597210)
• W. Becker, R. Blatt, G. Werth Precise Determination of 135Ba+ and 137Ba+ Hyperfine Structure, Journ. de Physique Colloque C8 339 (1981), (ID: 619682)

1979

Article

• R. Blatt, U. Schmeling, G. Werth On The Sensitivity of Ion Traps for Spectroscopic Applications, Appl. Phys. 20 295 (1979), (ID: 619685)