here are several ways to make twin particles. The ‘easy way is with

Twins here are several ways to make twin particles. The ‘easy way is with a laser and a nonlinear crystal. A beam of ultraviolet photons enters the crystal and about one in a billion times they interact with quantum fluctuations in the crystal lattice to create two red photons. This is known as ‘spontaneous down conversion. There are two types of down conversion. In a type I interaction the twins have the same polarization, and in a type II they are at 90 degrees to each other. You can set up the experiment with either type, but it is important to remember which you used, or you will easily become confused. When we talk about photon experiments, we are usually referring to type I interactions because they are easier to understand. Often the actual experiment uses oppositely polarized photons because they are easier to generate. Polarization is a wavelike property of photons. You can visualize them wiggling up and down, side-to-side or something in between. We use polarization to our advantage when we go on holiday to the beach; light from the sun is randomly polarized, but when it glances off the ocean it becomes predominantly horizontally polarized. If we wear vertically polarized sunglasses the glare off the ocean is blocked and we can see the ocean more clearly. The following two pictures show this effect. The two photons we make with the crystal can be separated and sent to different places. The record so far is two towns near Geneva, 50 kilometers apart. For this experiment scientists ‘borrowed’ the unused fibers of Swisscom in the middle of the night - when phone traffic was light. A detector was placed at the end of each fiber to measure the particles. HOUSE_OVERSIGHT_016021