Balanced Homodyne Detection of Optical Quantum States at Audio-Band Frequencies and Below

TL;DR

This paper demonstrates a balanced homodyne detection method capable of measuring quantum states of light at audio-band frequencies, achieving flat shot-noise and significant squeezing detection crucial for gravitational wave detectors.

Contribution

It introduces techniques to mitigate low-frequency technical noise, enabling shot-noise measurements below 0.5 Hz and observing large squeezing across the audio-band.

Findings

Flat shot-noise achieved below 0.5 Hz

11.6 dB of shot-noise suppression observed

More than 10 dB squeezing detected down to 10 Hz

Abstract

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