Photon-level broadband spectroscopy and interferometry with two frequency combs

Dual-comb spectroscopy has emerged a powerful technique of Fourier transform spectroscopy without moving parts. Broad spectra can be acquired with a single photodetector in any spectral range where laser frequency combs are available. Because the technique is multiplex, systematic effects are minimized and a great consistency of the spectra can be achieved. We show that dual-comb spectroscopy can be implemented with photon-counting instrumentation and work at power levels one-billion-fold weaker than those usually employed. Our demonstration opens many scenarios for applications of this powerful spectroscopic technique.