  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.
