| We investigate the inherent nonlocality in quantum mechanics and the role of distinguishability in interference. When an ultraviolet photon interacts with a nonlinear optical medium, two correlated infrared photons can be produced. One can produce essentially single-photon states by triggering on one of the photons, and counting the other in coincidence. They may also be prepared in various entangled states, which are intrinsically nonlocal.; Subjecting one member of each pair to a cycle in polarization states in an interferometer has allowed the first observation of Berry's phase at the single photon level. When the interferometer is set outside the white-light fringe regime, the results can be interpreted in terms of a nonlocal "collapse of the wavefunction". In a quantum eraser experiment, an interfering system is first rendered incoherent by making the alternate paths which contribute to the overall process distinguishable. By "erasing" the which-path information after the output port of the interferometer, one may recover interference upon coincidence detection. Experimental results are given and three new quantum eraser schemes are proposed.; In an experiment proposed by Franson, each of the down-converted photons is sent into an unbalanced interferometer. We were able to observe sinusoidal coincidence fringes with visibilities of up to 90%, while the classical limit for such fringes is only 50%. Moreover, the data violate an appropriate Bell's inequality, which limits the correlations permitted by any theory constrained by "local realism". Here the elements of reality that are refuted are the energy and time of emission of the photons. We infer that Nature is inherently nonlocal.; To perform an incontrovertible test of this sort is very difficult, and has not yet been done; one reason is that detection efficiencies need to be quite high. To this end, we have measured the absolute quantum efficiencies (and other operating characteristics) of several single-photon detectors. Efficiencies as high as 76.4 {dollar}pm{dollar} 2.3% were seen, with implications that efficiencies as high as 90% may be achievable. Finally, a possible source for a loophole-free test of Bell's inequalities is presented. |
