  Interference of photons emerging from independent sources is essential for
modern quantum information processing schemes, above all quantum repeaters and
linear-optics quantum computers. We report an observation of non-classical
interference of two single photons originating from two independent, separated
sources, which were actively synchronized with an r.m.s. timing jitter of 260
fs. The resulting (two-photon) interference visibility was 83(+/-)4 %.
