# How is Entanglement Created?

1. Jun 21, 2012

### referframe

Say we have a system consisting of two electrons, traveling freely, that are not initially entangled, either in spin or in position-momentum.

Then, because of their electric fields, a photon is emitted and absorbed between the two electrons.

Are the electrons now guaranteed to be at least partially entangled in position-momentum?

2. Jun 21, 2012

### San K

Interesting question. They are not guaranteed to be entangled, not even partially.

that said...I know very little about it, will find out more. looking forward to some great answers from the forum.

3. Jun 22, 2012

### spoirier

If you look for a process that can produced entangled particles, a natural one is to take a pair of electrons and separate them passively in a spatial manner without using a magnetic field. Then their spins are entangled (opposite to each other in any direction of measurement). Still it is not used for experiments at long distance because it is not practical to transport electrons and they can lose their spin by interaction with photons that are usually present in the form of thermic radiation (does anyone know at which speed for given temperatures ?). Also it may be hard to produce as pairs of electrons are present in atoms but don't easily get out. Shooting away the kernel of an helium atom is possible but, well, not easy. Experiments were done with entangled photons but I personally don't know the production method.

Then, it is useless to discuss partial entanglements as quantum theory does not distinguish a blurred entanglement from a classical correlation : a entangled pair of electrons produced as above that has 1/3 probability to be preserved and 2/3 probability to be blurred (replaced by uncorrelated random spins) is the same quantum system as a classical correlated pair produced by a 1/3 probability of having been produced by a classical correlation in each of the 3 dimensions. In other words, by throwing a dice to decide if the spins are (up,down), (down,up), (right,left), (left,right), (face,back) or (back,face).

Last edited: Jun 22, 2012
4. Jun 22, 2012

### Zarqon

Not sure what you mean by a photon being emitted and absorbed by electrons. Free electrons are not bound systems, and so do not have excited states that can absorb photons.

But if you mean that a photon is scattered off the electrons, then the key property which causes them to become entangled is that something is "indistinguishable". Meaning, if a photon scatters near both electrons (within the wavelength) such that it is not possible to tell which of the two electrons it scattered off of, then they become entangled.

5. Jun 23, 2012