# How to entangle two particles? [duplicate]

This question already has an answer here:

After learning about quantum entanglement I wanted to know, what is the simplest way to entangle two particles in a lab?

## marked as duplicate by ACuriousMind♦, JamalS, Brandon Enright, Kyle Kanos, Waffle's Crazy PeanutDec 8 '14 at 6:25

• Possible duplicate: How does one produce entanglement? – Gigi Butbaia Dec 7 '14 at 19:23
• @Gigi Butbaia: your answer to that question doesn't address the question. The question was how to take two non-entangled particles, and entangle them. But, please see my comment to that answer. – Sofia Dec 8 '14 at 0:41

One of the answers to "How does one produce entanglement?" gives an example of how to obtain pairs of entangled particles. However, it is a complicated example. The simplest procedure, and widely used in experiments, is down-conversion of ultraviolet (UV) photons. Such photons are sent on a type of crystal that we name "non-linear crystal". In short, inside the crystal, the UV photon is split into two photons, one of them we name signal photon, the other idler photon. This topic is covered in Wikipedia under the name Spontaneous Parametric Down Conversion.

However, I believe that you ask how can we take two particles, and obtain an entangled pair.

Here is a simple procedure: send upon a 50-50% beam-splitter (BS) two photons of identical wavelength, but have polarization perpendicular to one another, e.g. one photon polarized in the direction x, and the other one in the direction y. Select only the cases in which at each output of the BS we have one photon (the selection may be done later by checking if we obtained two photons in coincidence).

The two photons are in the state

|S> = (1/sqrt(2)) { $|x>_A$ $|y>_B$ + $|y>_A$ $|x>_B$ } ,

where A and B are the two outputs.

Good luck !

• Sofia, in response to your flag: the best thing to do when you expect that a question will be closed as a duplicate is to post your answer to the question that will be linked in the duplicate message. So I recommend that you delete this answer and repost it to there. Posting an answer to an old question bumps it to the top of the front page so people will see it. (For future reference, when you have questions like this, ask them on Physics Meta or in Physics Chat, not in a flag.) – David Z Dec 8 '14 at 16:52