Publications Christoph R. KAUBRüGGER

2022

Article

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 (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.

2021

Article

C. R. Kaubrügger, D. Vasilyev, T. Schulte, K. Hammerer, P. Zoller Quantum Variational Optimization of Ramsey Interferometry and Atomic Clocks, Phys. Rev. X 11 41045 (2021-12-06), http://dx.doi.org/10.1103/PhysRevX.11.041045 doi:10.1103/PhysRevX.11.041045 (ID: 720629) Toggle Abstract
We discuss quantum variational optimization of Ramsey interferometry with ensembles of N entangled atoms, and its application to atomic clocks based on a Bayesian approach to phase estimation. We identify best input states and generalized measurements within a variational approximation for the corresponding entangling and decoding quantum circuits. These circuits are built from basic quantum operations available for the particular sensor platform, such as one-axis twisting, or finite range interactions. Optimization is defined relative to a cost function, which in the present study is the Bayesian mean square error of the estimated phase for a given prior distribution, i.e. we optimize for a finite dynamic range of the interferometer. In analogous variational optimizations of optical atomic clocks, we use the Allan deviation for a given Ramsey interrogation time as the relevant cost function for the long-term instability. Remarkably, even low-depth quantum circuits yield excellent results that closely approach the fundamental quantum limits for optimal Ramsey interferometry and atomic clocks. The quantum metrological schemes identified here are readily applicable to atomic clocks based on optical lattices, tweezer arrays, or trapped ions.

Invited Talk

C. R. Kaubrügger • Quantum Variational Optimization of Ramsey Interferometry and Atomic Clocks, Perspectives on Quantum Sensing and Computation for Particle Physics (online) URL (2021-07-08), (ID: 720751)

Seminar Talk

C. R. Kaubrügger • Quantum Variational Optimization of Ramsey Interferometry and Atomic Clocks, Q-SEnSE seminars (online) (2021-03-05), URL (ID: 720750)

Talk

C. R. Kaubrügger • Quantum Variational Optimization of Ramsey Interferometry and Atomic Clocks, Joint Annual Meeting of ÖPG and SPS (Innsbruck, 2021-08-30) URL (2021-08-31), (ID: 720752)

2019

Article

C. R. Kaubrügger, P. Silvi, C. Kokail, R. van Bijnen, A. M. Rey, J. Ye, A. Kaufman, P. Zoller Variational spin-squeezing algorithms on programmable quantum sensors, Phys. Rev. Lett. 123 260505 (2019-08-22), http://dx.doi.org/10.1103/PhysRevLett.123.260505 doi:10.1103/PhysRevLett.123.260505 (ID: 720356) Toggle Abstract
Arrays of atoms trapped in optical tweezers combine features of programmable analog quantum simulators with atomic quantum sensors. Here we propose variational quantum algorithms, tailored for tweezer arrays as programmable quantum sensors, capable of generating entangled states on-demand for precision metrology. The scheme is designed to generate metrological enhancement by optimizing it in a feedback loop on the quantum device itself, thus preparing the best entangled states given the available quantum resources. We apply our ideas to generate spin-squeezed states on Sr atom tweezer arrays, where finite-range interactions are generated through Rydberg dressing. The complexity of experimental variational optimization of our quantum circuits is expected to scale favorably with system size. We numerically show our approach to be robust to noise, and surpassing known protocols.