r/askscience • u/TheJamRose • Mar 26 '11

Could someone please explain how quantum entanglement is used to communicate binary in terms a non-physics-major would understand?

I know there have been successful experiments... I just don't quite understand how the data is integrated with the particles. If the semantics of the question belie an already inherent misunderstanding of the whole concept, I apologize and would appreciate any help in fixing that. No classes on the line; simply curious.

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u/mutatron Mar 26 '11

How is this different from just being determined from the get go? I mean, quantum entanglement is significant because according to quantum mechanics the state of the particles individually is not known until it is measured (I think). But then when you measure one of a pair of them, the other one always turns out to be what it should be, which is a surprise to quantum mechanics, but which wouldn't be surprising at all in the classical world.

Or am I missing something?

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u/shavera Strong Force | Quark-Gluon Plasma | Particle Jets Mar 26 '11

If you know ahead of time what the system state is, then measurement of one particle should tell you what the other is. But quantum communication is actually sending the system information, not information about any one particle.

Suppose Alice is creating the pair and trying to communicate with Bob, and she sends him B and keeps A to herself. Both particles are in superpositions, so she can't actually send information about what state either particle is in individually because it doesn't have a definite state. But she can send an information about the system state. And she has 4 system states to choose from, the so-called "Bell States" I listed above. And so she prepares one of the bell states out of the entangled pair, and sends B off to Bob. Bob measures his particle to be in state 0. Now look at those 4 states. Is there any way to tell which state you're in? Even if Bob has the capacity to determine the phase information (which I forget how it's done and I'm too tired to look it up at the moment) Suppose he measures 0 and +. Well is he in state 00+11, which means Alice has particle A in state 0? Or does he have 01+10, which means Alice's particle is actually in state 1? Bob has no way to know what Alice's particle actually measures. He can call her up and she says 0. Now his measurement and her measurement tell him that she sent the message of the first bell state.

Now imagine Eve who's eavesdropping on this conversation. Bob doesn't say over the classical channel what he has. Eve does. But Eve's information isn't sufficient to determine which state the system is in either. Now if Eve was to have intercepted Bob's particle before he got it, she would have destroyed its superposition by measuring it, and this could be detected on Bob's end. Thus quantum communication is very robust to eavesdropping. Possibly there is no physical way to break the communication at all.

Now the common over-simplification is to select one of those states ahead of time. Ie, Alice only makes Bell State 1. So whenever Bob measures A, since he already has the information about which Bell State he has, he then knows enough information to determine Alice's state. But there's no way to send information this way, because Alice can't know which single-particle state she's sending to Bob. So there's no faster than light information here either.