Quick question on ensembles of pure states

[metroplex021]

Hi folks,

I just have a question concerning whether it is possible to produce an ensemble of individual electrons in pure single-particle spin states. I'm assuming it is possible, but sometimes one hears that strictly speaking all electrons are entangled with one another, which would mean that they are not individually in a pure state -- so I"m a bit confused about this! I'd also like to hear how it is possible to produce such an ensemble (if it is indeed possible) physically.

Thanks very much!

 

[jfizzix]

A beam of electrons can indeed be polarized, so that it is an ensemble of pure spin states. Whether or not those states stay pure depends on the strength of the interaction between the spins of the electrons.

I expect this interaction is small for the densities and time scales usually considered (a beam of electrons in a vacuum is a lot more diffuse than electrons crowded around the nucleus of an atom).

If the spins do appreciably interact, then the state of the group of electrons becomes entangled if you wait long enough. It could also be that the interaction happens slowly enough that for the time it takes an electron to travel from source to detector, it's state doesn't have time to change much.

 

[metroplex021]

Perfect. Thanks very much!