: Does anybody know where I can find some info on photon entanglement. This is for a seminar in physics class so I would like to know how exactly you entangle a photon.

Please discard previous message with so many spelling errors. Yes. There are certain nonlinear (BBO) crystals, such as are used in optical parametric oscillators, that will supply entangled photon pairs. They are known as photon down converters. Down conversion is a phenomenon known to microwave waveguide researchers/engineers. Most of these crystals are quite hygroscopic and degrade with use requiring repolishing. There are a number of compounds that yeild nonlinearity. A rare earth is typically one of the components. Basically what happens is: a photon of wavelength x enters the crystal and two photons of wavelength 2x exit. This of course conserves the total energy equation. The two photons are "entangled". The detection of one implies the existence of the other. This can be used to obtain "which way" information for the "other" photon without "touching" it. Some very interesting experiments and devices can be contrived as a result. Discourgements: 100% entaglement (photons out / photons in) is never obtained. The crystals are used in an mild oven to keep them dry. They are expensive. Some of the experimets imply time reversal so you'll draw skeptics like flies. Encouragements: The experimental results are always in line with Quantum Electrodynamics and Maxwell equations. New insights into Nature may be gleaned. This is one of the early steps in understanding the MIT teleportation experiment.