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In quantum optics an ensemble of photons is treated as a Bose condensate which has a de Broglie wavelength given by l 0 ͞N where l 0 and N are the wavelength and average number of constituent photons, respectively. We describe an interferometer which is capable of measuring this de Broglie wavelength. We show specifically that a coherent state input has a de Broglie coherence length proportional to 1͞ p N. Finally we show that the interferometer is Heisenberg limited in sensitivity (Df min 1͞N) and thus forms a new class of interferometers operating at the fundamental quantum limit.

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In this letter we experimentally implement a single photon Bell test based on the ideas of S. Tan et al. [Phys. Rev. Lett., 66, 252 (1991)] and L. Hardy [Phys. Rev. Lett., 73, 2279(1994]. A double heterodyne measurement is used to measure correlations in the Fock space spanned by zero and one photons. Local oscillators used in the correlation measurement are distributed to two observers by co-propagating it in an orthogonal polarization mode. This method eliminates the need for interferometrical stability in the setup, consequently making it a robust and scalable method.

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A metal-clad optical waveguide with a semiconductor microcavity structure is proposed to increase the coupling efficiency of spontaneous emission into a lasing mode (spontaneous emission coefficient P) and to increase a total spontaneous emission rate simultaneously. Such a microcavity semiconductor laser with enhanced spontaneous emission has novel characteristics, including high quantum efficiency, low threshold pump rate, broad modulation bandwidth, and intensity noise reduced to below the shot-noise limit (amplitude squeezing).

An operational measure to quantify the sizes of some "macroscopic quantum superpositions", realized in recent experiments, is proposed. The measure is based on the fact that a superposition presents greater sensitivity in interferometric applications than its superposed constituent states. This enhanced sensitivity, or "interference utility", may then be used as a size criterion among superpositions.

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