  We report an interference experiment of spontaneous emission of light from
two distant solid-state ensembles of atoms that are coherently excited by a
short laser pulse. The ensembles are Erbium ions doped into two LiNbO3 crystals
with channel waveguides, which are placed in the two arms of a Mach-Zehnder
interferometer. The light that is spontaneously emitted after the excitation
pulse shows first-order interference. By a strong collective enhancement of the
emission, the atoms behave as ideal two-level quantum systems and no which-path
information is left in the atomic ensembles after emission of a photon. This
results in a high fringe visibility of 95%, which implies that the observed
spontaneous emission is highly coherent.
