# Ask a Physicist Answers

Hello, I am trying to figure out the practice behind the experiment I've heard about in a National Geographic documentary, so please help me a bit.

How do they generate entangled particles in the lab (photons)? Also, how do they make photons to interact with each other and what kind of interaction do they make?

**—Asked by Ivan from Croatia**

Entanglement can be thought of as a set of pieces that are correlated in a special way. When two particles are entangled, measuring something about one tells you what you would measure about the other, even though you don't look at the other.

As a really simple example, consider two identical twins who share just two shirts, one black and one white. They get dressed and one twin gets on the bus. You see her and notice the black shirt. You know exactly what color shirt her sister is wearing — white — even though you don't see her. Of course, there's a rule you need to know — the twins will always wear opposite shirts.

For particles, the rules are often quite simple and follow basic laws of physics such as momentum conservation, energy conservation, and angular momentum conservation.

In the example of photons, the momentum is just the energy divided by the speed of light with the direction specified. Also note that photons have ONE unit of angular momentum or spin (an intrinsic property held by elementary particles), and because they are massless, their spin is either along the momentum or opposite.

One way to get correlated photons (or in this case gamma-rays) comes from electron-positron annihilation (positrons are antiparticles of electrons). When an electron and positron annihilate at rest, the result is energy in the form of two photons.

A bubble chamber in which an electron-positron pair collided, producing two photons.**Image Credit:** CERN

Energy and momentum conservation require that these two photons go in opposite directions, and each has exactly half of the energy created: 0.511 MeV. Angular momentum conservation tells us that the two photon spins must also cancel. What this means is that if one photon, say the one that goes to the left, has its spin PARALLEL to the momentum, the other photon must also have its spin parallel to the momentum. (A particle with spin PARALLEL to its momentum is called RIGHT HANDED, OPPOSITE is called LEFT HANDED).

The two spins are opposite just as the momenta are opposite. So if we have a way to measure the spin of a photon, we only need to measure one to know exactly the state of the other. There are two photons and each photon can be R or L, but there are only two possible combinations: RR & LL. The combinations RL and LR are not possible. These are called entangled states.

Another way to get correlated visible photons is from a process in special materials called down conversion. In these materials, the electric field of the photons causes the electrons in the material to oscillate around their normal positions in a way that results is TWO photons with half the energy but moving in the same general direction as the incident photon. The linear polarizations of these down-conversion photons is also entangled, that is if one is vertical, the other is horizontal.

**Answered by:**

Tim Chupp, PhD

Professor of Physics

University of Michigan