  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.
