Broadband sensitivity improvement and back-action evasion via coherent quantum feedback with PT-symmetry

Conventional resonant detectors are subject to bandwidth-peak sensitivity trade-off, which can be traced back to the quantum Cramer-Rao Bound. Anomalous dispersion has been shown to improve it by signal amplification while leading to instability. We propose a stable quantum amplifier enabled by two-mode non-degenerate parametric amplification. Operated at the threshold, one amplifier mode is PT-symmetric to the original detector mode. Our scheme is applicable to all linear systems operating at fundamental limits. Sensitivity improvements are shown for laser-interferometric gravitational-wave detectors and microwave cavity axion detectors. For gravitational-wave detectors, we further proposed a more complete PT-symmetry structure to include the test mass and, therefore, to compensate for the measurement backaction.