Partial distinguishability in femtosecond optical spontaneous parametric down-conversion

Quantum interference is observed using femtosecond parametric down-conversi on in the absence of spectral filtering of the down-converted light. The we ll-known symmetric dip observed in cw polarization-interferometry experimen ts becomes less pronounced and substantially asymmetric under such pulsed e xcitation. We show that this asymmetry arises from the partial distinguisha bility of contributions from space-time segments within the down-conversion crystal. We develop a theory that achieves agreement with experiment by co herently adding intrasegment contributions while incoherently adding inters egment contributions.