For instance I have spontaneous parametric downconversion source,
how do I set a proper coincidence window to obtain correct coincidence count rate. I saw people often use 2 ns window time, even they have used different crystal operated at different wavelength. So I wonder how to choose the right coincident time window, is it related to the phase matching bandwidth in case of SPDC?
Hi Bo,
For SPDC without a cavity, the phasematching bandwidth is almost always large enough that the coherence length of the photons is much less than the detector timing resolution. Thus the coincidence window is limited by the detector and electronic jitter rather than the photon bandwidth. A good way to set the coincidence window is to build a coincidence timing histogram or start-stop measurement, where the start is one photon and the stop is the other. The width of the coincidence peak in the timing histogram then gives you a good starting point for the coincidence window.
Best regards,
Evan