a) The process of spontaneous parametric down-conversion (SPDC). Inside a nonlinear crystal, two outgoing photons are created from an incoming photon. Momentum and energy conservation apply, resulting in a characteristic emission cone. b) Two photons created by SPDC encounter polarizers. Depending on the polarization, the photons are either absorbed or transmitted. After passing the polarizers, the photons are collected with two optical fibers and detected with single photon counters. The detector indicates a coincidence when both photons arrive within roughly 30 ns.