Loss-Tolerant Detection of Squeezed States in the 2  μm Region

Squeezed states of light enable quantum-enhanced measurements but are limited by optical loss, particularly at 2  μm where photodiode efficiency is low. We report the first loss-tolerant, audio-band squeezed light detection at 1984 nm by using a phase-sensitive amplifier to amplify the squeezed vacuum prior to detection. This technique increases the effective detection efficiency from 74% to 95% and increases the observed squeezing from 4 to 8 dB, the highest level of squeezing observation reported at this wavelength. Additionally, the vacuum to dark-noise clearance increases, extending the effective measurement bandwidth toward lower frequencies. This approach is largely wavelength independent, extending high-fidelity quantum measurements to future gravitational-wave detectors and related quantum technologies.