# Photon source for Bell test

Wiki tells us that In practice most actual experiments have used light, assumed to be emitted in the form of particle-like photons (produced by atomic cascade or spontaneous parametric down conversion), rather than the atoms that Bell originally had in mind. The property of interest is, in the best known experiments, the polarisation direction

What device can actually be used for that source which creates a photon pair with entangled polarisations?

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The process used in this kind of source is the spontaneous parametric down conversion (SPDC, see, e.g. Wikipedia for details). It is a nonlinear optical process in which from a photon with angular frequency $\omega_0$ you get two photons with frequencies $\omega_1$, $\omega_2 = \omega_0-\omega_1$. These photons are then phase matched and have correlated polarization (either the same or opposite, we speak of type I SPDC and type II SPDC, respectively).