A conventional resonant detector is often subject to a trade-off between bandwidth and peak sensitivity that can be traced back to quantum Cramer-Rao Bound. Anomalous dispersion has been shown to improve it by signal amplification and is thus more robust against decoherence, while it leads to instabilities. We propose a stable quantum amplifier applicable to linear systems operating at the fundamental detection limits, enabled by two-mode non-degenerate parametric amplification. At threshold, one mode of the amplifier forms a PT-symmetric system of original detector mode. Sensitivity improvements are shown for laser-interferometric gravitational-wave detectors and microwave cavity axion detectors.
@article{arxiv.2012.00836,
title = {Broadband sensitivity improvement via coherent quantum feedback with PT symmetry},
author = {Xiang Li and Maxim Goryachev and Yiqiu Ma and Michael E. Tobar and Chunnong Zhao and Rana X Adhikari and Yanbei Chen},
journal= {arXiv preprint arXiv:2012.00836},
year = {2020}
}