How To Entangle Particles?

[obione]

I'm trying to understand entanglement but was curious about the actual physical process of entangling particles? How is this done in the lab? could you consider the double slit experiment as a tangling device for photons for example? And lastly, what particles can be entangled?

Cheers..

[clem]

The double slit involves only one particle at a time, so cannot lead to entanglement of two particles.
Entanglement occurs when two particles are produced in a decay where the spin of one is correlated with the spin of the other.

[vanesch]

It's very simple to entangle particles: have them interact ! Now, for photons, that's hard, because photons mostly don't interact (I know I know, you can have higher-order box diagrams in QED, but that's not what we are talking about here). So there are only very special processes which can entangle photons. The only way is to create them entangled, or to have something interact with both of them before decoherence sets in.

However, it is much harder to *observe* entangled systems, because decoherence sets in very soon. The trouble is that if your particles interact easily with stuff, then they entangle easily with stuff, and in order to observe entanglement between two particles, you don't want a third one to entangle with them, because if this happens, the "test" for entanglement between the two will fail. This is in fact how decoherence is supposed to work: *too much* entanglement kills (observable) entanglement.
This is also why we've only been able to do entanglement experiments on things like photon pairs : other particles interact too easily with the environment and the test wouldn't work. But if they don't interact easily, it is also not easy to have them get entangled in the first place.

So we have two problems:
- if particles don't interact, it is hard to get them entangled.
- if particles interact, you can easily entangle them (just let them interact), but then they easily interact with other stuff too, and this makes entanglement experiments fail.

[DrChinese]

How is this done in the lab?

The most common technique is call Spontaneous Parametric Down Conversion (often abbreviated SPDC or just PDC). A laser fires a beam of photons into a special crystal (often Barium Borate). Most of the time, photons go straight through and cannot be used. But occasionally, say one in a million times, an individual photon "splits" into a pair of photons. The pair emerge off angle slightly and at a different wavelength than the input beam. Conservation of total momentum, spin and energy apply, and therefore they are entangled. This pair can then be routed to the experiment for further use.

You can read more here: Spontaneous Parametric Down Conversion (http://en.wikipedia.org/wiki/Spontaneous_parametric_down_conversion)

[obione]

However, it is much harder to *observe* entangled systems, because decoherence sets in very soon. The trouble is that if your particles interact easily with stuff, then they entangle easily with stuff, and in order to observe entanglement between two particles, you don't want a third one to entangle with them, because if this happens, the "test" for entanglement between the two will fail. This is in fact how decoherence is supposed to work: *too much* entanglement kills (observable) entanglement.

Firtly, thanks for all the replies. Based on above, if you assume to have a pair of highly interacting particles(You want to be able to entangle them easily) that are entangled, would it be possible to say send them of in opposite diretions then trap them in some sort of circular magnetic field (cyclotron)? How long before decoherence sets in?

[vanesch]

Firtly, thanks for all the replies. Based on above, if you assume to have a pair of highly interacting particles(You want to be able to entangle them easily) that are entangled, would it be possible to say send them of in opposite diretions then trap them in some sort of circular magnetic field (cyclotron)? How long before decoherence sets in?

If the particles are charged, then decoherence sets in extremely quickly (I don't have numbers handy but there are estimates of this) - indeed, it is sufficient that one single other particle "sensed" the presence of a charge for this to be the case. Of course, if the trajectories are the same, you can still hope to entangle just the spin states, which might survive a bit longer before they decohere. But usually, decoherence sets in very quickly.

[sudhirking]

Why does this happen... why do the photons split ther angular momentum equally. Is it just because it is light?o, then wah tproperties of light makes it like this.