# Paired particles

1. May 22, 2004

### Ahmes

hello,
can anyone tell me what's the story about "paired particles" (e.g. paired photons)? I've heard about the "quantum encryption" technique, which uses a couple of photons who have some sort of link between them.
I am currently a high school student (learning physics), and in school we learned about the quantum aspect of wave interference - that the single photon turns randomly to one of the holes, but statistically, many photons act as we expect them to. I guess it has something to do with quantum encryprion.

help anyone?

thank...
Ahmes

2. May 22, 2004

### chroot

Staff Emeritus
If you bounce two particles off each other, they become "entangled." Entanglement means that the two particles must be in complementary states. For example, if one of the particles is spin-up, the other must be spin-down to conserve angular momentum. The interesting part is that you can't really say whether a particle is spin-up or spin-down until you measure it. When that measurement occurs on one of the particles, the other one might be all the way across the galaxy! Somehow the other particle has to know about the measurement of the first, even if it is very far away. In quantum mechanics, both particles are part of a single system; a measurement on any part of that system affects the entire system all at once, causing the other particle to immediately assume it's correct, complementary state.

- Warren

3. May 23, 2004

### Ahmes

Sorry for my ignorance

Thank you very much for the reply.
you said that this thing happens when you "bounce two particles off each other". what exactly does that mean? Does it have anything to do with a photon becoming an electron and a positron? In which sort of experiment can you "entangle" two particles?

thanks again.

4. May 23, 2004

### chroot

Staff Emeritus
If you collide two particles, you'll have an entangled pair. There may be other ways to prepare them, but I mostly have seen collisions.

- Warren

5. May 24, 2004

### ZapperZ

Staff Emeritus
Actually, in most EPR-type experiment involving entangled photons so far have used the spontaneous parametric down conversion to generate those photons. You have one incoming photon that excite a system. The system then decays via a two-step process, generating two photons that together have the same polarization as the original incoming photon.

More description here:

http://arxiv.org/abs/quant-ph/0106113

There are certainly other ways to get entangled particle. A cooper pair in a superconductor is one candidate. If you break a pair apart without destroying its coherence, they will form an entangled pair.

Zz.