Four-femtosecond propagation time measurement on single photons and nonlocal cancellation of group velocity dispersion

Abstract

In the process of spontaneous parametric down-conversion, conjugate pairs of broadband photons are emitted simultaneously from a nonlinear crystal. We use coincidence detection in a fourth-order interferometer to measure the propagation delay of one photon with respect to that of its conjugate. One photon of every pair enters each arm of the interferometer, and when the path-length difference is adjusted such that the two photons meet essentially simultaneously at the output beam splitter, a dip is observed in the coincidence rate between photon counters placed at the two output ports. In our experiment, the rms width of this dip is 15 fs. By placing various dielectric media in the path of one of the photons and then adjusting the free-space path lengths until the coincidence dip reappears, we have measured the propagation time of single photons and found it to agree with predictions based on the group velocity.1 Because of a surprising quantum-mechanical effect, group-velocity dispersion cancels out, such that the expected wave packet broadening has essentially no effect on the high time resolution of this technique.2