Question: How difficult is it to entangle two of your atoms? How about four?

Chris Mansell answered on 18 Jun 2013:

The easiest way we entangle the atoms in the lab where I work is by putting them into an entangle state called W state. We do this in the following way.

We trap several atoms in the same place. (This technique works for all different numbers of atoms up to a maximum number where we just can’t fit any more atoms in.) We then send in a laser beam that excites one of the atoms into an excited state. However, we don’t know which atom was excited and even in principle, there is no way of telling. To make this answer not take up too much text, I will just consider two atoms from now on. The first possibility for these two atoms is “atom 1 is excited and atom 2 isn’t” and the second possibility for these two atoms is “atom 2 is excited and atom 1 isn’t.” In quantum mechanics, both possibilities occur at the same time. This is very odd but this is our best understanding of what the theory is telling us. The state I have described (with the two possibilities occuring at the same time) is an entangled state.

In practice, this is quite difficult because it takes a lot of playing with lasers, magnetic fields, optical components, etc. Anyway, this is what we are trying to do.