Restoring dispersion cancellation for entangled photons produced by ultrashort pulses - art. no. 053810

It is a well-known and remarkable fact that in certain coincidence photon-c ounting experiments with cw-pumped parametric down-converters, the effects of group-velocity dispersion arising from media interposed between source a nd detectors are completely canceled, even if the media physically affect o nly one of the photons of the pair. Recently Perina et al. [Phys. Rev. A 59 , 2359 (1999)] showed that this phenomenon does not occur when certain clas sical timing information is available about the arrival of individual photo ns at the detectors, as is the case when the photon pairs are produced via spontaneous parametric down-conversion using an ultrashort pump pulse. In t his paper we show that the nonlocal cancellation of dispersion far such a s ource of entangled photons can be restored in principle by proper engineeri ng of the source properties. In particular, we describe techniques for reco vering interference in coincidence-counting experiments by suppressing dist inguishing information without the post selection of photons. Moreover, a p recise classical timing signal coincident with the photon pair is still ava ilable.