Two-photon entanglement in type-II parametric down-conversion

We present an experimental study of the entangled two-photon polarization states in type-II optical parametric down-conversion. It is interesting to see that the polarization entanglement depends on the detection spectral bandwidth, i.e., the bandwidth of the spectral filters placed in front of the detectors. The entanglement is also dependent on the length of the down-conversion crystal. A simple quantum model is provided to explain these phenomena.