Partial Distinguishability in Femtosecond Optical Spontaneous Parametric Down-Conversion

Quantum interference is observed using femtosecond parametric down-conversion in the absence of spectral filtering of the down-converted light. The well-known symmetric dip observed in cw polarization-interferometry experiments becomes less pronounced and substantially asymmetric under such pulsed excitation. We show that this asymmetry arises from the partial distinguishability of contributions from space-time segments within the down-conversion crystal. We develop a theory that achieves agreement with experiment by coherently adding intrasegment contributions while incoherently adding intersegment contributions.