Amplification of 2um squeezed light

Abstract

Squeezed states of light enable the reduction of quantum noise in precision interferometric measurements and are routinely employed in gravitational-wave detectors. Future gravitational-wave detector upgrades based on cryogenic silicon optics are expected to operate at longer wavelengths near 1984nm, to take advantage of reduced coating thermal noise and substrate absorption. However, at these wavelengths, the observable squeezing is severely limited by optical loss, in particular the reduced quantum efficiency of available photodiodes.

This thesis presents the development and experimental demonstration of a loss-tolerant detection scheme for squeezed states at 1984nm. A phase-sensitive optical parametric amplifier is implemented as a pre-amplifier, amplifying the squeezed quadrature prior to photodetection. A theoretical framework is developed to describe the generation, amplification, and detection of squeezed states in the presence of optical loss and phase noise, introducing the concept of effective detection efficiency for amplified readout. The model predicts that phase-sensitive amplification can mitigate detection loss without significantly degrading the quantum correlations of the squeezed state.

Experimentally, a squeezed-light source operating at 1984nm is employed, consisting of second-harmonic generation for pump production and an optical parametric oscillator for squeezed vacuum generation. A phase-controlled optical parametric amplifier is added to the squeezed light output for amplified readout. The individual subsystems are characterized in terms of cavity parameters, escape efficiency, nonlinear gain, and control-loop performance. Amplified squeezing is demonstrated with substantially improved effective detection efficiency compared to conventional homodyne readout, despite the limited photodiode quantum efficiency. The technique also enhances noise clearance above the electronic noise floor, extending the usable measurement bandwidth.

These results establish phase-sensitive amplification as a practical strategy for overcoming detection loss at long wavelengths and demonstrate its relevance for next-generation gravitational-wave detectors designed for the 2um regime.