Controllable Frequency Entanglement via Auto-Phase-Matched Spontaneous   Parametric Down-Conversion
  A new method for generating entangled photons with controllable frequency correlation via spontaneous parametric down-conversion (SPDC) is presented. The method entails initiating counter-propagating SPDC in a single-mode nonlinear waveguide by pumping with a pulsed beam perpendicular to the waveguide. The method offers several advantages over other schemes, including the ability to generate frequency-correlated photon pairs regardless of the dispersion characteristics of the system. Numerical evidence demonstrates the improvement provided by this source in the special case of frequency-correlated two-photon states. 
Zachary D. Walton, Mark C. Booth, Alexander V. Sergienko, Bahaa E.A.   Saleh, Malvin C. Teich