Enhancing Heralding Efficiency and Biphoton Rate in Type-I Spontaneous   Parametric Down-Conversion
  The nonlinear optical process of spontaneous parametric down-conversion (SPDC) is widely studied for applications in quantum information science due to its ability to produce two photons that can be entangled in many degrees of freedom. For applications in quantum communication, two metrics of this process are particularly important: heralding efficiency and total joint rate. Here, we derive expressions for both quantities for a variety of different beam geometries and frequencies. We pay specific attention to the spectrum of both biphotons and individual photons. We reveal the underlying mechanisms responsible for the spectral shape and show they differ for different geometries and frequencies. We then use these spectra to calculate heralding efficiency and joint count rate and examine how each of these metrics changes with different geometries, frequencies, and spectral filtering and beam parameters. Interestingly, we find very high heralding efficiencies are achievable for collinear geometries, while noncollinear geometries require spectral filtering for high heralding efficiency, ultimately limiting the joint count rate. We also find that the the spectrum is narrower in nondegenerate cases, leading to lower joint count rates and higher heralding efficiency in the noncollinear case. In addition to the theory, we verify selected predictions with experimental results. 
Hannah E. Guilbert and Daniel J. Gauthier
quant-ph physics.optics