  Two-photon excitation spectroscopy with broad spectral span is demonstrated
at Doppler-limited resolution. We describe first Fourier transform two-photon
spectroscopy of an atomic sample with two mode-locked laser oscillators in a
dual-comb technique. Each transition is uniquely identified by the modulation
imparted by the interfering comb excitations. The temporal modulation of the
spontaneous two-photon fluorescence is monitored with a single photodetector,
and the spectrum is revealed by a Fourier transform.
