Exposing gravitational waves below the quantum sensing limit

The sensitivities of ground-based gravitational-wave (GW) detectors are limited by quantum shot noise at a few hundred hertz and above. Nonetheless, one can use a quantum-correlation technique proposed by Martynov et al. [Phys. Rev. A 95, 043831 (2017) to remove the expectation value of the shot noise, thereby exposing underlying classical signals in the cross spectrum formed by cross-correlating the two outputs in a GWinterferometer's antisymmetric port. We explore here the prospects and analyze the sensitivity of using quantum correlation to detect astrophysical GW signals. Conceptually, this technique is similar to the correlation of two different GW detectors as it utilizes the fact that a GWsignal will be correlated in the two outputs but the shot noise will be uncorrelated. Quantum correlation also has its unique advantages as it requires only a single interferometer to make a detection. Therefore, quantum correlation could increase the duty cycle, enhance the search efficiency, and enable the detection of highly polarized signals. In particular, we show that quantum correlation could be especially useful for detecting postmerger remnants of binary neutron stars with both short (< 1 s) and intermediate (similar to 10-10(4) s) durations and setting upper limits on continuous emissions from unknown pulsars.