# Multiple quantum entanglements

1. May 3, 2012

### serp777

Lets say you have a particle A, which then physically interacts with particle C and B to become entangled. Since A is entangled with B, and C is entangled with A, then it would follow that c is entangled to B. I guess my main question is it possible to have multiple systems entangled with each other simultaneously, or does it have to be binary? And my next question is if its possible, then is C technically entangled to B, even though they are entangled through A?

2. May 3, 2012

### DrChinese

You can have entanglement of more than 2 particles, there is no real absolute upper limit other that practical ones due to decoherence. So A, B and C form an entangled system and you would not say that A and B are entangled and A and C are entangled. On the other hand, if you observe the spin of A, then you throw B and C into a different entangled state (2 particles instead of 3).

3. May 4, 2012

### zonde

You can't have entanglement between A/B and A/C at the same time. You can have so called GHZ state but it gives less "entanglement per photon" figuratively speaking. Basically you detect one of the three photons and then from that result you can find out what type of correlation you should expect for other two photons.

As a loose analogy we could use this - single equation with two unknowns (entanglement) and single equation with three unknowns (GHZ state).

4. May 4, 2012

### StevieTNZ

I don't think that works. Photons B and C would be in a definite state (unless they're entangled with other photons). Interaction between B and A wouldn't cause entanglement in this case, so far as what I've been told by Markus Arndt.

Perhaps if B and C were entangled, and A was entangled with another photon D the best you can get from interaction between B (or C) and A is an entanglement swap. This would create entanglement between D and B (or C), and A and C (or B).