  We demonstrate single-photon time-domain interference in a new realm. We
observe interferences in the photon counting statistics with two separate
mode-locked femtosecond lasers of slightly different repetition frequencies,
each emitting a comb of evenly spaced spectral lines over a wide spectral span.
We exploit the interference pattern for spectroscopic diagnostics over a broad
spectral range. An experimental proof-of-concept shows that the emerging
technique of high-resolution dual-comb Fourier transform spectroscopy can be
performed at light powers that are a billion-fold weaker than those commonly
employed. Our experiments challenge the intuitive concept that a photon exists
before detection and they open the prospect of precise spectroscopy over broad
spectral bandwidth in light-starved conditions.
