# Quantum entanglement in communications possible?

1. ### Moviemann345

24
Is it possible to move an entangled particle, without disrupting the wave function, to create a communication signal?

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3. ### Andrey

14
well surely if a particle is entangled it's already displaying signs of 'communication' since what happens to one paticle will instantaniously affect the other???

4. ### Tomsk

227
There is no way to send information FTL if that's what you're wondering. Any interaction with a particle in an entangled states tends to break the entanglement, but even then there is no possible way for the two particles to exchange information, so you can't use them for communication. You can however do some very cool things like secure signaling, teleportation, dense coding and stuff, but nothing that violates relativity in any way.

5. ### Andrey

14
Hold on a sec, why is interaction with a particle that is entangled, breaks the entanglement?

6. ### Tomsk

227
When you just look at one particle in an entangled pair (Bell state), it appears to be in something called a maximally mixed state, which means it has a 50/50 chance of being found spin up or down along ANY axis. But a measurement projects it into a pure state, and in a pure state you can always find an axis along which the probabilities are 1 and 0 or 0 and 1 for spin up/down. So after a measurement, the particle can no longer be in a mixed state, so the entanglement is broken.

7. ### Andrey

14
Can you simplify by what you mean a mixed state? Are you refering to particle-wave-function property or something else?

8. ### vanhees71

3,265
A mixed state is used to describe a system whose state is not completely determined. A mixed state is described by a positive semidefinite self-adjoint operator $$\hat{R}$$ with $$\text{Tr} \hat{R}=1$$, the Statistical operator of the system.

A pure state is a particular case of this more general situation. $$\hat{R}$$ describes a pure state if it is a projection operator with $$\hat{R}^2=\hat{R}$$. Then there exists a normalized state ket $$|\psi \rangle$$ such that $$\hat{R}=|\psi \rangle \langle \psi |$$.

Now take a two-spin system in the entangled pure state

$$|\Psi \rangle=\frac{1}{\sqrt{2}} [|\mathrm{up},\mathrm{down} \rangle - \mathrm{down},\mathrm{up} \rangle]$$.

Then the spin of particle 1 is described by the reduced Statistical operator

$$\hat{R}=\text{Tr}_2 |\Psi \rangle \langle \Psi |=\frac{1}{2} \hat{1}$$

which is the state of maximal ignorance with respect to the von Neumann entropy as the information measure.

Last edited: Jul 30, 2010
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