Pure single photon generation by type-I PDC with backward-wave amplification

Abstract

Linear optical quantum computing (LOQC) schemes, such as single photon quantum gates, require sources of pure single photons. Such single photon sources may be realized via photon pair generation by parametric downconversion (PDC). In this process pump photons inside a medium with a χ(2)-nonlinearity decay into two photons typically labelled signal and idler. The photon number correlation between the resulting fields can be exploited to herald the existence of one photon by detection of its partner. However the purity of the heralded photon is limited by correlations within each photon pair arising from energy and momentum conservation between pump, signal and idler photons. It has been shown that these correlations may be eliminated either by spectral filtering of the generated photon pairs or by group velocity matching [1]. The effective downconversion rate can be considerably improved by waveguided PDC. However in order to generate separable photon pairs in waveguides, either extremely narrow filter bandwidths are needed or the photon pair generation is limited to very specific wavelengths and materials by group velocity matching.