Simultaneous measurement of group and phase delay between two photons

We report on an experiment to determine both the group and phase delays experienced by orthogonally polarized photon pairs traveling through a birefringent medium. Both types of delay are determined from the same set of coincidence-counting data. The experiment is based on an interference technique using two-photon multipath indistinguishability to produce an interference feature. Earlier work has shown that this interference feature can be used to measure the group velocity of single-photon wave packets in dielectric media. In the current: work, the two-photon interferometer has been modified to produce an additional interference feature that is sensitive to the phase velocity of the light. We have used this technique to simultaneously measure the group delay in crystal quartz with a precision of 0.1 fs and the phase delay with a precision of 8 attoseconds. Our analysis clarifies the effects of group and phase delays and shows the unexpected result that dispersive temporal broadening, which is well known to be canceled for the original interferometer setup, is not canceled for this type of ''postponed compensation'' interferometer.