Institute for Molecular Engineering, University of Chicago, 5640 South Ellis Avenue, Chicago, IL, 60637, USA.
Nat Commun. 2018 Oct 17;9(1):4320. doi: 10.1038/s41467-018-06477-7.
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 nonreciprocity is a powerful resource for sensing: it allows one to exceed the fundamental bounds constraining any conventional, reciprocal sensor.
非厄米系统的非常规性质，如存在异常点，最近被提议作为一种传感资源。然而，在量子领域中，噪声和效用的影响仍不清楚。在这项工作中，我们分析了由有效非厄米哈密顿量描述的线性耦合模式系统的参数传感特性。我们的分析充分考虑了经典和量子领域中的噪声效应，并且还充分考虑了基于相干驱动和外差探测的实际和最优测量协议。我们专注于双模器件，为任何这样的传感器推导了信号功率和信噪比的基本限制。我们使用这些来证明增强的信号功率需要增益，但不一定需要接近异常点。此外，当包括噪声时，我们表明非互易性是传感的有力资源：它允许一个人超过限制任何常规互易传感器的基本限制。
