Violations of a new inequality for classical fields

Two entangled photons incident upon two distant interferometers can give a coincidence counting rate that depends nonlocally on the sum of the phases of the two interferometers. It has recently been shown that experiments of this kind may violate a simple inequality that must be satisfied by any classical or semi-classical field theory. The inequality provides a graphic illustration of the lack of objective realism of the electric field. The results of a recent experiment which violates this inequality and in which the optical path length between the two interferometers was greater than 100 m are briefly described

Nonclassical Nature of Dispersion Cancellation and Nonlocal Interferometry

Several recent papers have shown that some forms of dispersion cancellation have classical analogs and that some aspects of nonlocal two-photon interferometry are consistent with local realistic models. It is noted here that the classical analogs only apply to local dispersion cancellation experiments [A.M. Steinberg et al., Phys. Rev. Lett. 68, 2421 (1992)] and that nonlocal dispersion cancellation [J.D. Franson, Phys. Rev. A 45, 3126 (1992)] is inconsistent with any classical field theory and has no classical analog. The local models that have been suggested for two-photon interferometry are shown to be local but not realistic if the spatial extent of the interferometers is taken into account. It is the inability of classical models to describe all of the relevant aspects of these experiments that distinguishes between quantum and classical physics, which is also the case in Bell's inequality.Comment: 10 pages, 8 figures; minor revisions, to appear in Phys. Rev.

Dispersion cancellation and non-classical noise reduction for large photon-number states

Nonlocal dispersion cancellation is generalized to frequency-entangled states with large photon number N. We show that the same entangled states can simultaneously exhibit a factor of 1/sqrt(N) reduction in noise below the classical shot noise limit in precise timing applications, as was previously suggested by Giovannetti, Lloyd and Maccone (Nature v412 (2001) p417). The quantum-mechanical noise reduction can be destroyed by a relatively small amount of uncompensated dispersion and entangled states of this kind have larger timing uncertainties than the corresponding classical states in that case. Similar results were obtained for correlated states, anti-correlated states, and frequency-entangled coherent states, which shows that these effects are a fundamental result of entanglement.Comment: 8 pages, 4 figures, REVTeX 4, submitted to Phys. Rev. A, v2: minor changes in response to referee report, fig3 fixe

Optically enhanced production of metastable xenon

Metastable states of noble gas atoms are typically produced by electrical discharge techniques or "all-optical" excitation methods. Here we combine electrical discharges with optical pumping to demonstrate "optically enhanced" production of metastable xenon (Xe*). We experimentally measure large increases in Xe* density with relatively small optical control field powers. This technique may have applications in systems where large metastable state densities are desirable.Comment: 3 pages, 3 figure

Cyclical Quantum Memory for Photonic Qubits

We have performed a proof-of-principle experiment in which qubits encoded in the polarization states of single-photons from a parametric down-conversion source were coherently stored and read-out from a quantum memory device. The memory device utilized a simple free-space storage loop, providing a cyclical read-out that could be synchronized with the cycle time of a quantum computer. The coherence of the photonic qubits was maintained during switching operations by using a high-speed polarizing Sagnac interferometer switch.Comment: 4 pages, 5 figure