Non-Hermitian Quantum Sensing: Fundamental Limits and Non-Reciprocal Approaches

TL;DR

This paper investigates the fundamental limits of non-Hermitian quantum sensors, demonstrating that non-reciprocity enhances sensing capabilities beyond traditional bounds without relying on exceptional points.

Contribution

It provides a comprehensive analysis of noise effects in non-Hermitian quantum sensing and shows non-reciprocity as a key resource for surpassing conventional sensing bounds.

Findings

Enhanced signal power requires gain, not necessarily exceptional points.

Non-reciprocity allows sensors to exceed fundamental bounds of reciprocal systems.

Optimal measurement protocols are based on coherent driving and homodyne detection.

Abstract

Unconventional properties of non-Hermitian systems, such as the existence of exceptional points, have recently been suggested as a resource for sensing. The impact of noise and utility in quantum regimes however remains unclear. In this work, we analyze the parametric-sensing properties of linear coupled-mode systems that are described by effective non-Hermitian Hamiltonians. Our analysis 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 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 sensing: it allows one to exceed the fundamental bounds constraining any conventional, reciprocal sensor. We analyze simple two-mode non-reciprocal sensors that allow this parametrically-enhanced sensing, but which do not involve exceptional point physics.