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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)
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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.
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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)
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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.
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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)
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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.
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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)
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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.
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M. K. Joshi Applications of variational methods and quantum simulation with trapped ion chains,
Group seminar: Department of nuclear and atomic physics (Tata Institute of Fundamental Research, Mumbai, 2023-08-07) (2023-08-08),
(ID: 721116)
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A trapped ion system is one of the most promising platforms for quantum computation, simulation, precision measurements, and quantum networks. In such a system, one can achieve a high level of control which is essential for solving hard-to-compute physics problems with very high accuracy. In particular, quantum simulation of complex physics models is one of the direct applications of such systems [1,2,3]. With the combination of single and two-qubit gates, we can implement a variety of Hamiltonians and simulate the time dynamics of a desired initial state in such systems. A very robust and efficient method is implementing a variational ansatz to optimize experimental parameters such that one can prepare a desired quantum state, which can be either the ground state or an excited state of the Hamiltonian of interest. In this seminar, I will present quantum simulation results obtained by Floquet engineering [4] and variational methods [5,6]. I will present experimental results on the characterization of entanglement in our platform and finally provide an outlook on the trapped ion quantum simulator.
[1] Joshi et al. "Observing emergent hydrodynamics in a long-range quantum magnet." Science 376.6594 (2022): 720-724.
[2] Blatt et al. "Quantum simulations with trapped ions." Nature Physics 8.4 (2012): 277-284.
[3] Joshi, Manoj K., et al. "Quantum information scrambling in a trapped-ion quantum simulator with tunable range interactions." Physical Review Letters 124.24 (2020): 240505.
[4] Kranzl et al. "Experimental observation of thermalization with noncommuting charges." PRX Quantum 4.2 (2023): 020318.
[5] Kokail et al. "Self-verifying variational quantum simulation of lattice models." Nature 569.7756 (2019): 355-360.
[6] Joshi et al. "Exploring Large-Scale Entanglement in Quantum Simulation." arXiv preprint arXiv:2306.00057 (2023).
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M. K. Joshi Trapped ion systems: controlling ions for quantum simulation and computation,
Group seminar: Department of nuclear and atomic physics (Tata Institute of Fundamental Research, Mumbai, 2023-08-07) (2023-08-07),
(ID: 721115)
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In the past few decades, technical advancements in controlling quantum systems with high fidelity have triggered worldwide hope in realizing fully controllable quantum computers and simulators, which aim to achieve quantum advantages. The leading systems are superconducting qubits, quantum dots, ultra-cold atoms, and trapped ions. In particular, trapped ion systems have a great level of isolation from surrounding perturbations and coherent control can be achieved with a high level even for long ion strings. In this seminar, I will give a brief introduction to ion trapping, laser cooling, and initialization of qubits in long ion chains [1,2]. Furthermore, I will describe single and two-qubit control on ion chains and discuss techniques to perform quantum computation and simulation work [3,4].
[1] Hrmo et al. "Sideband cooling of the radial modes of motion of a single ion in a Penning trap." Physical Review A 100.4 (2019): 043414.
[2] Joshi et al. "Polarization-gradient cooling of 1D and 2D ion Coulomb crystals." New Journal of Physics 22.10 (2020): 103013.
[3] Kranzl et al. "Controlling long ion strings for quantum simulation and precision measurements." Physical Review A 105.5 (2022): 052426.
[4] Joshi et al. "Observing emergent hydrodynamics in a long-range quantum magnet." Science 376.6594 (2022): 720-724.
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M. K. Joshi Controlling Long Ion Strings for Quantum Simulation,
Group seminar: David Lukas group (University of Oxford, 2023-07-28) (2023-07-28),
(ID: 721114)
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In this seminar, I will present how a long ion string is coherently controlled and used for analog quantum simulation experiments in Innsbruck. I will discuss our single ion addressing setup and present methods to simulate various Hamiltonians in our analog quantum simulator platform. In our setup, individual qubit control is achieved with a far-detuned laser beam that interacts with a specific ion of interest and imprints an AC Stark shift [1]. A combination of global and local rotations is used for preparing an arbitrary product state that goes an input state for quantum simulation experiments and is also used for projecting the experimentally realized quantum state onto arbitrary bases. Power law decaying entangling interactions between trapped ions are generated by shining a global laser beam from the radial direction, which couples the electronic and radial motional degrees of freedom. By taking advantage of single and two-qubit control, we simulate a variety of Hamiltonians and study the transport of spin excitations in our 51-qubit simulator [2].
[1] Kranzl, Florian, et al. "Controlling long ion strings for quantum simulation and precision measurements." Physical Review A 105 (5), 052426.
[2] Joshi, Manoj K., et al. "Observing emergent hydrodynamics in a long-range quantum magnet." Science 376 (6594), 720-724.