Optical noise correlations and beating the standard quantum limit in advanced gravitational-wave detectors

The uncertainty principle, applied naively to the test masses of a laser-interferometer gravitational-wave detector, produces a standard quantum limit (SQL) on the interferometer's sensitivity. It has long been thought that beating this SQL would require a radical redesign of interferometers. However, we show that LIGO-II interferometers, currently planned for 2006, can beat the SQL by as much as a factor two over a bandwidth Deltaf similar to f, if their thermal noise can be pushed low enough. This is due to dynamical correlations between photon shot noise and radiation-pressure noise, produced by the LIGO-II signal-recycling mirror.