Type II downconversion and polarisation entanglement

This lecture covers the wave equation in nonlinear medium, second harmonic generation, three wave mixing processes, phase matching vs. quasi-phase matching, and the generation of polarisation entanglement through spontaneous parametric down-conversion. It also discusses the critical vs. noncritical phase matching, issues with bulk nonlinear crystals, and the advantages of using waveguides for efficient twin photon flux generation. Additionally, it explains quasi phase matching using periodically poled crystals and its applications in quantum nonlinear optics.