Dr. Hoi-Kwan Lau Profile

Dr. Hoi-Kwan Lau

at Simon Fraser Univ

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Proceedings Article | 9 March 2022 Presentation

Reservoir-engineered spin squeezing: macroscopic even-odd effects and hybrid-systems implementations

Martin Koppenhoefer, Peter Groszkowski, Hoi-Kwan Lau, Aashish Clerk

Proceedings Volume PC12016, PC120160Q (2022) https://doi.org/10.1117/12.2616978

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We revisit the dissipative approach to producing and stabilizing spin-squeezed states of an ensemble of N two-level systems, providing a detailed analysis of two surprising yet generic features of such protocols. The first is a macroscopic sensitivity of the steady state to whether N is even or odd. We discuss how this effect can be avoided (if the goal is parity-insensitive squeezing), or could be exploited as a new kind of sensing modality to detect the addition or removal of a single spin. The second effect is an anomalous emergent long timescale and a "prethermalized" regime that occurs for even weak single-spin dephasing. We also discuss a general hybrid-systems approach for implementing dissipative spin squeezing that does not require squeezed input light or complex multi-level atoms, but instead makes use of bosonic reservoir-engineering ideas. Our protocol is compatible with a variety of platforms, including trapped ions, NV defect spins coupled to diamond optomechanical crystals, and spin ensembles coupled to superconducting microwave circuits.

Proceedings Article | 5 March 2021 Presentation

Non-Hermitian quantum sensing: fundamental limits and non-reciprocal advantages

Hoi-Kwan Lau, Aashish Clerk

Proceedings Volume 11700, 117003J (2021) https://doi.org/10.1117/12.2586713

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Unconventional properties of non-Hermitian systems, such exceptional points, have recently been suggested as a resource for sensing. The impact of noise and utility in quantum regimes, however, remain highly debatable. In this talk, I will introduce a full theoretical framework to analyze the performance of a dispersive quantum non-Hermitian sensor; parts of our result have been included in our recent paper Lau & Clerk, Nat. Comm. 9, 4320 (2018). Our formalism fully accounts for noise effects in both classical and quantum regimes, and also fully treats a realistic and optimal measurement protocol based on coherent driving and homodyne detection. Focusing on two-mode devices, we derive fundamental bounds on the signal-to-noise (SNR) ratio for any such sensor. We use these to demonstrate that enhanced SNR ratio does not necessarily require any proximity to an exceptional point. Furthermore, we show that non-reciprocity is a powerful resource for sensing even when quantum noise exists.

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