# Example of Disentanglement?

1. Feb 12, 2013

### referframe

Given, say, two electrons whose spins are entangled in the singlet state. Over time, the two electrons have become somewhat spatially separated.

What would be an example of a physical experiment that disentangles the spins of two electrons?

2. Feb 12, 2013

### ZapperZ

Staff Emeritus
Maybe I misread this, but it isn't obvious that a simple scattering off one of the electrons would do this?

Zz.

3. Feb 12, 2013

### referframe

OK, but how does that work (mathematically) if the 2 spin-entangled electrons are now 100 miles apart?

4. Feb 12, 2013

### DrChinese

There is collapse, and of course distance is not a factor. It goes from being 2 particles making up a system in a shared state, to 2 particles now described by individual separable states.

Or are you asking the physical basis of that operation? (which no one knows...)

5. Feb 12, 2013

### Maui

It seems Heisenberg is more important than Newton in Nature's plans.

6. Feb 13, 2013

### San K

We now have two separate (individual) wave-functions instead of one single wave function (earlier) to describe the particles.

7. Feb 13, 2013

### ZapperZ

Staff Emeritus
Do you know what to do if they are only 1 micron apart?

There's something I am not understanding with this whole thread. Why do you require a distance? Why electrons in particular? Why are you not getting that the very act of measurement itself destroys the entanglement? There are things here that made the original question rather baffling.

Zz.

8. Feb 13, 2013

### kith

As you probably know, a simple spin measurement "disentangles" them.

I guess you are looking for a continuous process of "disentangeling" which doesn't involve the measurement problem. I think one example would be the experimental realization of the Jaynes-Cummings model of quantum optics. Here, you have a particle and a mode of the electrical field which get entangled and disentangled periodically.

It isn't obvious to me, why a state |Bell state of an EPR pair> × |state of a scattering partner> should generally evolve such that the EPR pair is described by a product state afterwards.

9. Feb 13, 2013

### ZapperZ

Staff Emeritus
Er... there's nothing that says that it HAS to be a "product state afterwards", at least in experiments.

I can show you an experiment in which just ONE single scattering event is sufficient induce decoherence:

https://www.physicsforums.com/showpost.php?p=1498616&postcount=55

Zz.

10. Feb 13, 2013

### kith

Ok, but somehow the degree of entanglement has to be reduced. Being not very familiar with scattering, this is still not obvious to me.

This is interesting, thanks for the link! But having read only the abstract, I don't see how it applies to this thread. If I get it right, they start with an electron in superposition and let it interact with an additional electron. This leads to momentum entanglement between the two electrons and the disappearance of the interference fringes of the first electron, i.e. decoherence occurs. How does this relate to the topic which is disentanglement of two electrons?

11. Feb 13, 2013

### ZapperZ

Staff Emeritus
Doesn't "disentanglement" imply decoherence? Entanglement requires maintaining coherence between the entangled entities. That's why photons are often used. They can maintain coherence over longer distances and time when compared to charged particles such as electrons.

Sure, there are varying degree of decoherence, but that experiment clearly shows that you only need an interaction with just ONE other particle for the original coherent state to be destroyed.

Zz.

12. Feb 13, 2013

### kith

No, you can also have a unitary time evolution from an entangled state to a product state. I think this is what happens in the Jaynes-Cummings model which I mentioned in my first post (and which has been realized experimentally in 1987 according to wikipedia). There, the entanglement is between a field mode and a particle.

I don't know if something similar has been done with two entangled particles. It's not hard to write down a proper quantum circuit but it may be very hard to realize it experimentally.

13. Feb 13, 2013

### ZapperZ

Staff Emeritus
.. and you think that this is what is meant by the OP when he mentioned "disentangles"?

Zz.

14. Feb 13, 2013

### kith

How should he know in advance? He doesn't know how to disentangle a system, so I told him about a second possibility besides the one which had already been mentioned. I'm a big fan of answering the actual question instead of offering a standard answer.

The problem with this thread is that the OP remains very vague so probably he won't benefit much from it.

15. Feb 14, 2013

### referframe

Thank you all for your input. I watched the entire 20-hour Leonard Susskind YouTube lectures on Quantum Entanglement and somehow he never directly associated measuring one of the spins as "disentanglement". In one lecture, he describes (non-mathematically) a situation in which particle A is entangled with particle B and then the experimenter throws particle B into a tub of water. He states that then particle A would be entangled with all the atoms in the tub of water and ONLY WHEN particle B is brought back close to particle A to interact with it does disentanglement occur.