Spatial distinguishability of photons produced by spontaneous parametric down-conversion with a focused pump
Ryan S. Bennink, Yun Liu, Duncan Earl, Warren P. Grice · Physical Review A · 2006
It is known that photons produced by spontaneous parametric down-conversion can be coupled into optical fibers more efficiently by focusing the pump field. We find that focusing the pump in type-II down-conversion causes photons of ordinary and extraordinary polarization to acquire very different angular spreads, which amounts to spatial information that distinguishes between the polarization states. Numerical studies predict that the photons collected by a detector or quantum channel will be of predominantly one polarization and that the degree of polarization entanglement will be lessened in some cases.