Experimental observation of double-peak structure of coincidence spectra   in ultra-fast Spontaneous Parametric Down Conversion
  The requirement on the symmetry of the biphoton wave function results in a double peak structure of the coincidence spectra of Spontaneous Parametric Down Conversion (SPDC) [1]. In this work we report the experimental observation of this effect by careful tailoring parameters of the SPDC crystal, the ultra-fast pump laser, and the measurement setup. The results are shown to be relevant to quantum state engineering of ultra-fast polarization-, or frequency-entangled mixed states. 
Dmitry A. Kalashnikov, Mikhail V. Fedorov, Leonid A. Krivitsky
quant-ph physics.optics