Shapiro recently suggested that nonlocal dispersion cancellation is essentially classical in nature. It is shown that his classical model gives identical dispersion in two beams of light and is not analogous to nonlocal dispersion cancellation.
Shapiro recently argued that nonlocal dispersion cancellation using entangled pairs of photons is essentially classical in nature, based on a comparison with a classical model in which two stationary, chaotic beams of light have phases and frequencies that are anticorrelated, which he refers to as ``phase-sensitive'' light [J. H. Shapiro, Phys. Rev. A 81, 023824 (2010)]. It is shown here that there is no physical cancellation of dispersion for classical light of that kind and Shapiro's results merely reflect the fact that identical dispersion occurs in both beams. The origin of the cross correlations between the intensities of the two beams is shown to be completely different in the classical and quantum mechanical cases.
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, P. G. Kwiat, and R. Y. Chiao, 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.
Franson's paradigm for nonlocal dispersion cancellation [J. D. Franson, Phys. Rev. A 45, 3126 (1992)] is studied using two kinds of jointly Gaussian-state signal and reference beams with phase-sensitive cross correlations. The first joint signal-reference state is nonclassical, with a phase-sensitive cross correlation that is at the ultimate quantum-mechanical limit. It models the outputs obtained from continuous-wave spontaneous parametric down-conversion. The second joint signal-reference state is classical---it has a proper P representation---with a phase-sensitive cross correlation that is at the limit set by classical physics. Using these states we show that a version of Franson's nonlocal dispersion cancellation configuration has essentially identical quantum and classical explanations except for the contrast obtained, which is much higher in the quantum case than it is in the classical case. This work bears on Franson's recent article [J. D. Franson, Phys. Rev. A 80, 032119 (2009)], which asserts that there is no classical explanation for all the features seen in quantum nonlocal dispersion cancellation.
Classically, two coincident light pulses propagating through two dispersive media will become broadened and their degree of coincidence will be reduced. When entangled photon pairs from parametric down-conversion are considered instead, it is found that the dispersion experienced by one photon can exactly cancel the dispersion experienced by the other in such a way that their coincidence is maintained. The dispersion cancellation is independent of the separation between the two photons and provides a further example of the nonlocal nature of the quantum theory.
Franson’s paradigm for nonlocal dispersion cancellation [J. D. Franson, Phys. Rev. A 45, 3126 (1992)] is studied using two kinds of jointly Gaussian-state signal and reference beams with phase-sensitive cross correlations. The first joint signal-reference state is nonclassical, with a phase-sensitive cross correlation that is at the ultimate quantum-mechanical limit. It models the outputs obtained from continuous-wave spontaneous parametric down-conversion. The second joint signal-reference state is classical—it has a proper P representation—with a phase-sensitive cross correlation that is at the limit set by classical physics. Using these states we show that a version of Franson’s nonlocal dispersion cancellation configuration has essentially identical quantum and classical explanations except for the contrast obtained, which is much higher in the quantum case than it is in the classical case. This work bears on Franson’s recent article [J. D. Franson, Phys. Rev. A 80, 032119 (2009)], which asserts that there is no classical explanation for all the features seen in quantum nonlocal dispersion cancellation.