Tunable control of the bandwidth and frequency correlations of entangled photons

We demonstrate experimentally a new technique to control the bandwidth and the type of frequency correlations (correlation, anticorrelation, and even uncorrelation) of entangled photons generated by spontaneous parametric downconversion. The method is based on the control of the group velocities of the interacting waves. This technique can be applied in any nonlinear medium and frequency band of interest. It is also demonstrated that this technique helps enhance the quality of polarization entanglement even when femtosecond pulses are used as a pump.