Quantum Entanglement - proven?

[Ward]

Hi,

I have some general questions on Quantum Entanglement?

1. Is there a maximum distance between the two objects before it does not work?
2. Has it actually been proven/tested? If so can anybody provide some further information on this?

Thanks,

Ward

 

[ShayanJ]

Hi

1. Is there a maximum distance between the two objects before it does not work?

No!

2. Has it actually been proven/tested? If so can anybody provide some further information on this?

Yes. Its being used in laboratories for testing different things.

Take a look at here.

 

[DrChinese]

Here is an experimental demonstration of entanglement over 144 km:

http://arxiv.org/abs/quant-ph/0607182

 

[StevieTNZ]

Remember all experiments to date haven't closed all the loopholes in one experiment; rather they have been closed in separate experiments (leaving open one or two of the other loopholes). Although I believe entanglement (non-local reality) does truly exist, these loopholes leave open the possibility of local reality - http://en.wikipedia.org/wiki/Loopholes_in_Bell_test_experiments

 

[bohm2]

Is there a maximum distance between the two objects before it does not work?

Not only is there no maximum distance but it has been argued that barring loopholes, if the non-local effects observed in Bell-type experiments propagate at any finite speed, then non-locality could be exploited for superluminal communication:

The new hidden influence inequality shows that the get-out won't work when it comes to quantum predictions. To derive their inequality, which sets up a measurement of entanglement between four particles, the researchers considered what behaviours are possible for four particles that are connected by influences that stay hidden and that travel at some arbitrary finite speed. Mathematically (and mind-bogglingly), these constraints define an 80-dimensional object. The testable hidden influence inequality is the boundary of the shadow this 80-dimensional shape casts in 44 dimensions. The researchers showed that quantum predictions can lie outside this boundary, which means they are going against one of the assumptions. Outside the boundary, either the influences can't stay hidden, or they must have infinite speed.

Quantum non-locality based on finite-speed causal influences leads to superluminal signalling
http://www.nature.com/nphys/journal/v8/n12/full/nphys2460.html
http://arxiv.org/pdf/1110.3795v1.pdf

 

[atyy]

Remember all experiments to date haven't closed all the loopholes in one experiment; rather they have been closed in separate experiments (leaving open one or two of the other loopholes). Although I believe entanglement (non-local reality) does truly exist, these loopholes leave open the possibility of local reality - http://en.wikipedia.org/wiki/Loopholes_in_Bell_test_experiments

I think of it a bit differently, using "proven" in two different senses. I would say that entanglement as a prediction of quantum mechanics has been "proven" in the same sense as the frame invariance of the speed of light as a prediction of relativistic quantum electrodynamics has been "proven". Here "proven" means that non-trivial predictions of the theory have been tested, and the theory has successfully predicted all results to date. "Proven" does not mean that the theory will not be falsified by future observations. The Bell tests are the most spectacular examples of entanglement, but things like the superconducting ground state and the fractional quantum hall state are also examples of entanglement.

Nonlocality, on the other hand is about accounting for experimental results independently of quantum mechanics, taking into account all possible theories beyond quantum mechanics. The spirit here is different and stricter, because the Bell inequalities are derived independently of quantum mechanics. Because I feel that here one should use a stricter notion of "proven", I would say nonlocality can never be proven, so here one typically tries to be precise by stating a null hypothesis, and stating p values.

Here's a talk by Subir Sachdev about current research on entanglement in high temperature superconductivity.

 

[bhobba]

Has it actually been proven/tested? If so can anybody provide some further information on this?

You have got some interesting answers here.

However I have got a slightly different take:
http://arxiv.org/pdf/0911.0695v1.pdf

Basically if you want to model physical processes probabilistically you have two choices - standard probability theory and QM.

What separates the two is one allows entanglement and the other doesn't - only QM allows entanglement. The other thing is only QM allows continuous transformations between pure states which is really required for physical processes. If a transformation can be applied for a second it can be applied for half a second an so on. So it turns out if you want this very reasonable requirement, and its hard to see how it can be done without, then you get entanglement.

Thanks
Bill

 

[ShayanJ]

However I have got a slightly different take:
http://arxiv.org/pdf/0911.0695v1.pdf

This one seems better than the previous papers where QM was merely a kind of probability theory!
The idea of pushing QM to become only a mathematical theory applied to the universe is too bold to be successful (also I don't like it:D), but this paper seems to take an appropriate compromise.