New high-efficiency source of photon pairs for engineering quantum entanglement

We constructed an efficient source of photon pairs using a waveguide-type nonlinear device and pel formed a two-photon interference experiment with an unbalanced Michelson interferometer. As the interferometer has two arms of different lengths, photons from the short arm on ive at the detector earlier than those from the long arm. We find that the arrival time difference (DeltaL/c) and the time window of the coincidence counter (DeltaT) are important parameters which determine the boundary between the classical and quantum regimes. Fringes of high visibility (80% +/- 10%) were observed when DeltaT < DeltaL/c. This result is explained only by quantum theory and is clear evidence fur quantum entanglement of the interferometer's optical paths.