Phase control of squeezed vacuum states of light in gravitational wave detectors

Quantum noise will be the dominant noise source for the advanced laser interferometric gravitational wave detectors currently under construction. Squeezing-enhanced laser interferometers have been recently demonstrated as a viable technique to reduce quantum noise. We propose two new methods of generating an error signal for matching the longitudinal phase of squeezed vacuum states of light to the phase of the laser interferometer output field. Both provide a superior signal to the one used in previous demonstrations of squeezing applied to a gravitational-wave detector. We demonstrate that the new signals are less sensitive to misalignments and higher order modes, and result in an improved stability of the squeezing level. The new signals also offer the potential of reducing the overall rms phase noise and optical losses, each of which would contribute to achieving a higher level of squeezing. The new error signals are a pivotal development towards realizing the goal of 6 dB and more of squeezing in advanced detectors and beyond.