Ultra-low phase noise squeezed vacuum source for gravitational wave detectors

Squeezed states of light are a valuable resource for reducing quantum noise in precision measurements. Injection of squeezed vacuum states has emerged as an important technique for reducing quantum shot noise, which is a fundamental limitation to the sensitivity of interferometric gravitational wave detectors. Realizing the most benefit from squeezed-state injection requires lowering optical losses and also minimizing squeezed quadrature fluctuations-or phase noise-to ensure that the large noise in the antisqueezed quadrature does not contaminate the measurement quadrature. Here, we present an audio band squeezed vacuum source with 1.3^+0.7 _-0.5 mrad of phase noise. This is a nearly tenfold improvement over previously reported measurements, improving prospects for squeezing enhancements in current and future gravitational wave detectors.