# How are atoms entangled and can it be done remotely?

#### xts

There are two cases.

1. Entangled spins (majority of entanglement experiments) - we may be pretty brutal to those particles, e.g. we may squeeze them into optical fibre - we may do whatever we like, just not changing their spin.

2. Entangled position/momentum - we never may get 100% entanglement, as our experiments are always spatially limited. Anyway pairs are created such, that sum of their momenta is fixed. It means they fly in different directions. We don't have to do anything special to separate them.

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#### DrChinese

Gold Member
I have a question that may be similar to what have discussed recently.
As we know to create an entangled state first of all we have to have an interaction (Although it's not sufficient as xts said in 19, but it seems that it's a necessary condition as Schrodinger's equation implys). Now my question is how it is possible that we separate these entangled particles far from each other so that they keep staying entangled?
I asked this question because theoretically if we want to separate the particles, we have to apply a potential (according to classical mechanics we need a force) that will change our previous Schrodinger's equation and so probably will disturb our previous entangled state.
Ah, but a lot of things don't collapse entanglement. For photons (as an example): mirrors, fiber, wave plates. Things that do not constitute a measurement will maintain entanglement.

#### hammertime

Very difficult.
But it's just a technical issue, right?

#### DrChinese

Gold Member
But it's just a technical issue, right?
Not sure, really, as it depends on exactly what you are trying to entangle (electrons, photons, etc and also what properties) and how far they are separated. There are a lot of severe constraints, which I think go beyond "technical".

#### hammertime

Not sure, really, as it depends on exactly what you are trying to entangle (electrons, photons, etc and also what properties) and how far they are separated. There are a lot of severe constraints, which I think go beyond "technical".
What are these severe constraints?

#### DrChinese

Gold Member
What are these severe constraints?
You would need to be more specific, as I mention. A hypothetical scenario:

There has been discussion of performing Bell tests on photons at large distance, perhaps even as far as the moon (using a mirror left there). That would show long distance entanglement. It is possible to store the entangled state of a photon for a period of time in a certain kind of lattice structure (IIRC), and then retrieve it. So perhaps that could be placed on the moon. Then you could have 2 entangled things far apart.

But that is probably the easiest of all the scenarios.

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