Balanced homodyne detection of optical quantum states at audio-band frequencies and below

The advent of stable, highly squeezed states of light has generated great\ninterest in the gravitational wave community as a means for improving the\nquantumnoise- limited performance of advanced interferometric detectors. To\nconfidently measure these squeezed states, it is first necessary to measure the\nshot-noise across the frequency band of interest. Technical noise, such as\nnon-stationary events, beam pointing, and parasitic interference, can corrupt\nshot-noise measurements at low Fourier frequencies, below tens of kilo-Hertz.\nIn this paper we present a qualitative investigation into all of the relevant\nnoise sources and the methods by which they can be identified and mitigated in\norder to achieve quantum noise limited balanced homodyne detection. Using these\ntechniques, flat shot-noise down to Fourier frequencies below 0.5 Hz is\nproduced. This enables the direct observation of large magnitudes of squeezing\nacross the entire audio-band, of particular interest for ground-based\ninterferometric gravitational wave detectors. 11.6 dB of shot-noise suppression\nis directly observed, with more than 10 dB down to 10 Hz.\n