  We demonstrate optical interferometry beyond the limits imposed by the photon
wavelength using 'triggered' entangled photon pairs from a semiconductor
quantum dot. Interference fringes of the entangled biphoton state reveals a
periodicity half of that obtained with the single photon, and much less than
that of the pump laser. High fringe visibility indicates that biphoton
interference is less sensitive to decoherence than interference of two
sequential single photons. The results suggest that quantum interferometry may
be possible using a semiconductor LED-like device.
