Extended phase-matching conditions for improved entanglement generation
  Extended phase-matching conditions for spontaneous parametric down-conversion are examined. By augmenting the conventional phase-matching conditions, they permit the creation of a class of frequency-entangled states that generalizes the usual twin-beam biphoton state. An experimental characterization of these states is possible through interferometric coincidence counting. 
Vittorio Giovannetti, Lorenzo Maccone, Jeffrey H. Shapiro, and Franco   N. C. Wong