Improving single-photon sources via linear optics and photodetection

In practice, single photons are generated as a mixture of vacuum with asingle photon with weights 1-p and p, respectively; here we are concerned withincreasing p by directing multiple copies of the single photon-vacuum mixtureinto a linear optic device and applying photodetection on some outputs toconditionally prepare single photon states with larger p. We prove that it isimpossible, under certain conditions, to increase p via linear optics andconditional preparation based on photodetection, and we also establish a classof photodetection events for which p can be improved. In addition we prove thatit is not possible to obtain perfect (p=1) single photon states via this methodfrom imperfect (p<1) inputs.Comment: 4 pages, 1 figur