  Coherent nonlinear spectroscopies and imaging in the X-ray domain provide
direct insight into the coupled motions of electrons and nuclei with resolution
on the electronic length and time scale. The experimental realization of such
techniques will strongly benefit from access to intense, coherent pairs of
femtosecond X-ray pulses. We have observed phase-stable X-ray pulse pairs
containing more thank 3 x 10e7 photons at 5.9 keV (2.1 Angstrom) with about 1
fs duration and 2-5 fs separation. The highly directional pulse pairs are
manifested by interference fringes in the superfluorescent and seeded
stimulated manganese K-alpha emission induced by an X-ray free-electron laser.
The fringes constitute the time-frequency X-ray analogue of the Young
double-slit interference allowing for frequency-domain X-ray measurements with
attosecond time resolution.
