Data from: Squeezed dual-comb spectroscopy

Laser spectroscopy and interferometry provide an unparalleled view into
the fundamental nature of matter and the Universe through ultra-precise
measurements of atomic transition frequencies and gravitational waves.
Optical frequency combs have expanded metrology capabilities by
phase-coherently bridging radio frequency and optical domains to enable
traceable high-resolution spectroscopy across bandwidths greater than
hundreds of terahertz. However, quantum mechanics limits the measurement
precision achievable with laser frequency combs and traditional laser
sources, ultimately impacting fundamental interferometry and spectroscopy.
Squeezing the distribution of quantum noise to enhance measurement
precision of either the amplitude or phase quadrature of an optical field
leads to significant measurement improvements with continuous wave lasers.
But experiments demonstrating true metrological advantage of broad
bandwidth squeezing with optical frequency combs are much less developed.
In this work, we generate bright amplitude-squeezed frequency comb light
and apply it to molecular spectroscopy using interferometry that leverages
the high-speed and broad spectral coverage of the dual-comb technique.
Using the Kerr effect in nonlinear optical fiber, the amplitude quadrature
of a frequency comb centered at 1560 nm is squeezed by >3 dB over a
2.5 THz of bandwidth that includes 2500 comb teeth spaced by 1 GHz.
Interferometry with a second coherent state frequency comb yields
mode-resolved spectroscopy of hydrogen sulfide gas with a signal-to-noise
ratio nearly 3 dB beyond the shot noise limit, taking full metrological
advantage of the amplitude squeezing when the electrical noise floor is
considered. The quantum noise reduction leads to a two-fold quantum
speedup in the determination of gas concentration, with impact for fast,
broadband, and high signal-to-noise ratio measurements of multiple species
in dynamic chemical and biological environments. Overall, this work
solidifies the general understanding of quantum noise reduction in Fourier
transform spectroscopy and motivates further exploration with squeezed
states in the broad application space of frequency comb metrology.