Optimal Ultra-wide Spatial-Spectral Windows for Hyperentangled   Two-photon Emission
  While being optimally compensated for spatial phase variations, the two-photon state produced by the two-crystal emission exhibits spatial and spectral decoherence off the central emission modes. In this paper, we present an experimentally convenient method to optimize the ultra-wide spatial and spectral windows; allowing the minimum spatial-spectral decoherence for a required two-photon flux. 
S. F. Hegazy, J. El-Azab, Y. A. Badr, and S. S. A. Obayya
quant-ph