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

Here we propose a new method of heralding pure single photons by the use of wave guided PDC with counter propagating photon-pairs: one travels with the pump beam while the other is back propagating towards the laser source. The wavelengths of these generated photon pairs, can be freely engineered by microstructured periodic poling of the chi-nonlinearity of the crystal and an appropriately chosen pump wavelength. This setup has profound advantages over the common method of creating copropagating photon pairs. Finally the backpropagating photon will exhibit a very narrow bandwidth ideally suited for long distance transmission in optical fibers. We present a theoretical treatment of counterpropagating PDC and a numerical analysis of the achievable purity for different materials and wavelength ranges. Our study demonstrates that, in this new regime of counter propagating PDC, highly nonlinear materials (such as PPLN) can be used to herald pure single photons over a broad frequency range.