# Example of Disentanglement?

by referframe
Tags: disentanglement
 PF Gold P: 133 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? As always, thanks in advance.
Emeritus
PF Gold
P: 29,243
 Quote by 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? As always, thanks in advance.
Maybe I misread this, but it isn't obvious that a simple scattering off one of the electrons would do this?

Zz.
PF Gold
P: 133
 Quote by ZapperZ Maybe I misread this, but it isn't obvious that a simple scattering off one of the electrons would do this? Zz.
OK, but how does that work (mathematically) if the 2 spin-entangled electrons are now 100 miles apart?

PF Gold
P: 5,441
Example of Disentanglement?

 Quote by referframe OK, but how does that work (mathematically) if the 2 spin-entangled electrons are now 100 miles apart?
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...)
P: 724
 Quote by referframe OK, but how does that work (mathematically) if the 2 spin-entangled electrons are now 100 miles apart?

It seems Heisenberg is more important than Newton in Nature's plans.
P: 915
 Quote by referframe OK, but how does that work (mathematically) if the 2 spin-entangled electrons are now 100 miles apart?
We now have two separate (individual) wave-functions instead of one single wave function (earlier) to describe the particles.
Emeritus
PF Gold
P: 29,243
 Quote by referframe OK, but how does that work (mathematically) if the 2 spin-entangled electrons are now 100 miles apart?
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.
P: 755
 Quote by referframe What would be an example of a physical experiment that disentangles the spins of two electrons?
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.

 Quote by ZapperZ Maybe I misread this, but it isn't obvious that a simple scattering off one of the electrons would do this?
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.
Emeritus
PF Gold
P: 29,243
 Quote by kith 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.
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:

http://www.physicsforums.com/showpos...6&postcount=55

Zz.
P: 755
 Quote by ZapperZ Er... there's nothing that says that it HAS to be a "product state afterwards", at least in experiments.
Ok, but somehow the degree of entanglement has to be reduced. Being not very familiar with scattering, this is still not obvious to me.

 Quote by ZapperZ I can show you an experiment in which just ONE single scattering event is sufficient induce decoherence.
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?
Emeritus
PF Gold
P: 29,243
 Quote by 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?
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.
P: 755
 Quote by ZapperZ Doesn't "disentanglement" imply decoherence?
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.
Emeritus
PF Gold
P: 29,243
 Quote by kith No, you can also have a unitary time evolution from an entangled state to a product state (or a state with a smaller degree of entanglement). 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 design a proper quantum circuit but it may be very hard to realize it experimentally.
.. and you think that this is what is meant by the OP when he mentioned "disentangles"?

Zz.
P: 755
 Quote by ZapperZ .. and you think that this is what is meant by the OP when he mentioned "disentangles"?
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.
 PF Gold P: 133 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.

 Related Discussions Quantum Physics 2 General Physics 0 General Physics 0 Quantum Physics 2