Non-Hermitian quantum sensing: exceptional point and non-reciprocal approaches

Unconventional properties of non-Hermitian systems, such as the existence of exceptional points, have recently been suggested as a resource for sensing [1,2]. The impact of noise and utility in quantum regimes however remains unclear. We describe here a full analysis of quantum parametric sensing using coupled mode systems described by effective non-Hermitian Hamiltonians; our approach rigorously accounts for quantum noise effects [3]. Focusing on two-mode devices, we derive fundamental bounds on the signal power and signal-to-noise ratio for any such sensor. We use these to demonstrate that enhanced signal power requires gain, but not necessarily any proximity to an exceptional point. Further, when noise is included, we show that non-reciprocity is a powerful resource for quantum sensing: it allows one to exceed the fundamental bounds constraining any conventional, reciprocal sensor. Non-reciprocal quantum sensors could be implemented in a variety of systems, including superconducting quantum circuits and quantum optomechanical systems.

[1] Hodaei et. al., Nature 548, 187–191 (2017)
[2] Chen et. al., Nature 548, 192–196 (2017)
[3] Hoi-Kwan Lau & Aashish A. Clerk, Nature Communications 9, 4320 (2018)