Description
Squeezed states of light are a key resource for quantum-enhanced metrology, enabling quantum noise reduction below the vacuum limit in the measured quadrature. However, the measurement enhancement provided by squeezing is highly sensitive to optical loss, which irreversibly degrades the quantum state by coupling in vacuum fluctuations. This limitation becomes increasingly important at wavelengths where detector materials and photodiode technology make high quantum efficiency difficult to realize.
In this work, building on an idea originally proposed by Caves [1], we investigate phase-sensitive amplification as a loss-tolerant readout technique for squeezed light at 2um, with particular relevance to next-generation gravitational-wave detectors. By amplifying the measured quadrature before photodetection, the effect of post-amplification optical loss is reduced, and the measured squeezing becomes limited primarily by propagation loss and the internal loss of the amplifier. We develop a theoretical model to predict the efficiency and limitations of this technique, including optical loss, finite amplifier escape efficiency, phase noise, and the effective detection efficiency of the overall measurement.
We realize this technique experimentally [2] by generating squeezed vacuum at 1984nm with an optical parametric oscillator, followed by amplification in a second nonlinear cavity operated as a phase-sensitive amplifier. With conventional homodyne detection, the observed squeezing is limited by the reduced quantum efficiency of available 2um photodiodes. With amplified readout, the measured squeezing increases from 4dB to 8dB, corresponding to an effective detection efficiency of approximately 95% using a photodiode with only 74% quantum efficiency. The amplification also raises the quantum noise above the electronic noise floor, extending the usable measurement bandwidth toward both lower and higher frequencies.
These results demonstrate phase-sensitive amplification as a practical approach for mitigating photodetection loss in quantum metrology.
[1] C. M. Caves, PRD 23, 1693 (1981).
[2] K. M. Kwan et al., PRL. 136, 123601 (2026).
| I am the presenting author | Yes |
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