Entangled photons – Experiments with entangled photons

Here we see a simplified representation of the experimental set-up. The aim of the experiment is to produce pairs of entangled photons.

BBO crystal: The crystal is optically birefringent. In this crystal, some of the incident ultra-violet photons, create pairs of two photons, each having half energy and reverse polarisation, i.e. once horizontally polarised, and once vertically.

Two polarisation rotators, two polarising beam splitters, a pair of detectors at Alice and Bobs side, respectively.

Alice and Bob each define their respective “vertical” and “horizontal” measuring axis by adjusting their polarisation rotators.
A huge number of ultraviolet photons run through the BBO crystal, without pair creation. Only with very small probability, pairs of entangled photons are created, leaving the crystal along the two red lines.