Exposing gravitational-wave signals below the shot noise with quantum correlation*

The sensitivities of ground-based gravitational wave (GW) detectors are limited by quantum shot noise at > a few x 100 Hz. Nonetheless, one can use a quantum correlation (QC) technique to remove the expectation value of the shot noise, thereby exposing underlying classical signals in the cross spectrum form by cross-correlating two photodiodes in a GW detector's antisymmetric port. The QC technique has been used to constrain the coating thermal noise in LIGO. Here we further explore the prospects and analyze the sensitivity of using the QC to detect GW signals, including both bursts with short and long durations and persistent GW emissions from pulsars with unknown locations. While conceptually similar to the correlation of two different interferometers, the QC has its unique advantages. In particular, with GC one can detect signals with only a single interferometer. Therefore, the QC technique does not require corrections to the differences in a signal's arrival time and antenna response which are necessary when cross-correlating two different detectors. This makes the QC computationally efficient when searching for sources with unknown locations.