Publications Matthias BOCK

2024

Article

• 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), URL (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 oscillator probing the system, thereby limiting the interrogation time and associated spectral resolution. Correlation spectroscopy overcomes this limitation by probing two quantum systems with the same noisy oscillator for a measurement of their transition frequency difference; this technique has enabled very precise comparisons of atomic clocks. Here, we extend correlation spectroscopy to the case of multiple quantum systems undergoing strong correlated dephasing. We model Ramsey correlation spectroscopy 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 derive precision limits and optimal sensing techniques for this problem and compare the performance of probe states and measurement with and without entanglement. Using one- and two-dimensional ion 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 qubit-resolved single-shot readout and is, thus, applicable to other physical systems such as neutral atoms in tweezer arrays.

2023

Articles

• I. Vybornyi, L. Dreissen, D. Kiesenhofer, H. Hainzer, M. Bock, T. Ollikainen, D. Vadlejch, C. F. Roos, T. E. Mehlstäubler, K. Hammerer Sideband thermometry of ion crystals, PRX Quantum 4 40346 (2023-06-14), http://dx.doi.org/10.1103/PRXQuantum.4.040346 doi:10.1103/PRXQuantum.4.040346 (ID: 721085) Toggle Abstract
  Coulomb crystals of cold trapped ions are a leading platform for the realisation of quantum processors and quantum simulations and, in quantum metrology, for the construction of optical atomic clocks and for fundamental tests of the Standard Model. For these applications, it is not only essential to cool the ion crystal in all its degrees of freedom down to the quantum ground state, but also to be able to determine its temperature with a high accuracy. However, when a large ground-state cooled crystal is interrogated for thermometry, complex many-body interactions take place, making it challenging to accurately estimate the temperature with established techniques. In this work we present a new thermometry method tailored for ion crystals. The method is applicable to all normal modes of motion and does not suffer from a computational bottleneck when applied to large ion crystals. We test the temperature estimate with two experiments, namely with a 1D linear chain of 4 ions and a 2D crystal of 19 ions and verify the results, where possible, using other methods. The results show that the new method is an accurate and efficient tool for thermometry of ion crystals.
• D. Kiesenhofer, H. Hainzer, A. Zhdanov, P. Holz, M. Bock, T. Ollikainen, C. F. Roos Controlling two-dimensional Coulomb crystals of more than 100 ions in a monolithic radio-frequency trap, PRX Quantum 4 20317 (2023-04-28), http://dx.doi.org/10.1103/PRXQuantum.4.020317 doi:10.1103/PRXQuantum.4.020317 (ID: 721039) Toggle Abstract
  Linear strings of trapped atomic ions held in radio-frequency (rf) traps constitute one of the leading platforms for quantum simulation experiments, allowing the investigation of interacting quantum matter. However, linear ion strings have drawbacks, such as the difficulty of scaling beyond approximately 50 particles as well as the inability to naturally implement spin models with more than one spatial dimension. Here we present experiments with planar Coulomb crystals of up to 105 40Ca+ ions in a novel monolithic rf trap, laying the groundwork for quantum simulations of two-dimensional spin models with single-particle control. We characterize the trapping potential by analysis of crystal images and compare the observed crystal configurations with numerical simulations. We further demonstrate stable confinement of large crystals, free of structural configuration changes, and find that rf heating of the crystal is not an obstacle for future quantum simulation experiments. Finally, we prepare the out-of-plane motional modes of planar crystals consisting of up to 105 ions close to their ground state by electromagnetically induced transparency cooling, an important prerequisite for implementing long-range entangling interactions.

2022

Posters

• Matthias Bock Correlation spectroscopy with multi-qubit-enhanced phase estimation in a planar 91-ion ion crystal, SFB BeyondC: Frontiers of Quantum Information Science (University of Vienna, 2022-09-04) (2022-09-04), (ID: 720921)
• Matthias Bock Correlation spectroscopy with multi-qubit-enhanced phase estimation in a planar 91-ion ion crystal, North American Conference on Trapped Ions (NACTI) (Duke University, 2022-08-01) (2022-08-01), (ID: 720920)

Preprint

• H. Hainzer, D. Kiesenhofer, T. Ollikainen, Matthias Bock, F. Kranzl, Manoj K. Joshi, Goni Yoeli, Rainer Blatt, Tuvia Gefen, C. F. Roos Correlation spectroscopy with multi-qubit-enhanced phase estimation, (2022-03-25), arXiv:2203.12656 arXiv:2203.12656 (ID: 720825) Toggle Abstract
  Precision spectroscopy on trapped-ion crystals subject to correlated dephasing can reveal a multitude of information in the absence of any single-particle coherences. We use correlation spectroscopy for measuring ion-ion distances and transition frequency shifts in one- and two-dimensional ion crystals by analyzing multi-particle correlations of up to N=91 qubits, each of which is encoded in a single ion. Additionally we carry out single-shot measurements of the laser-ion detuning and of path-length fluctuations. We show that the information contained in N-particle correlations reduces the measurement uncertainty as compared to the case where only two-particle correlations are analyzed. We derive quantum precision limits for this problem. While for a pair of qubits entanglement can reduce the measurement uncertainty, entanglement-free correlation spectroscopy becomes asymptotically optimal in the limit of N→∞ when the measurement goal is the determination of all phase differences between the qubits.

Talk

• Matthias Bock Multi-qubit-enhanced phase estimation in one- and two-dimensional ion crystals with up to 91 particles, International Conference on Quantum Optics (Obergurgl, 2022-02-20) (2022-02-20), (ID: 720922)