Squeezed dual-comb spectroscopy.

Optical frequency combs have enabled unique advantages in broadband, high-resolution spectroscopy and precision interferometry. However, quantum mechanics ultimately limits the metrological precision achievable with laser frequency combs. Quantum squeezing has led to significant measurement improvements with continuous wave lasers, but experiments demonstrating metrological advantage with squeezed combs are less developed. Using the Kerr effect in nonlinear optical fiber, a 1 GHz frequency comb centered at 1560 nm is amplitude-squeezed by >3 dB over a 2.5 THz bandwidth. Dual-comb interferometry yields mode-resolved spectroscopy of hydrogen sulfide gas with a signal-to-noise ratio nearly 3 dB beyond the shot-noise limit. The quantum noise reduction leads to a two-fold quantum speedup in the determination of gas concentration, with impact for high-speed measurements of multiple species in dynamic chemical environments.