Wanted: Quantum physicist, prepared to talk to person insane

[compilerbitch]

If I entangle two particles A and B in such a way that they are in the same quantum state, then entangle them such that they are in different quantum states, what happens?

1. The universe ends with the biggest core dump in the history of the multiverse

2. Both particles cease to exist in some kind of impressive explosively thrown exception

3. Both particles end up entangled to be in the same state

4. Both particles end up entangled to be in different states

5. Both particles end up entangled, but are randomly entangled as in 4. or 5. above

6. Entanglement is broken, with both particles left in random states

7. Doing what I said is impossible for some clever but not immediately obvious reason

8. I am asking a stupid question (though I'd argue that knowing exactly why it is stupid would be valuable)

(note that 5 and 6 are not equivalent if either particle ends up further entangled)

The same problem, from a quantum computing point of view, could be restated a bit more simply. Given N classical bits, the number of possible combinations of values is 2^N. With N qubits in a mutually entagled state, the number of possible combinations is strictly less than 2^N, such that if an observation (and collapse to a classical, non-superposed state) is forced, the resulting classical value is constrained by the original entangled state. My question is, what happens if the entangled state allows exactly zero possible classical values?

Comments

[hairyears.livejournal.com]

Entanglement means that an unmeasured property of A has a quantum counterpart in B. When A is measured, the 'wave function' collapses and measurement B will be the known counterpart of A, even if that information cannot possibly have been transmitted to 'B' after measurement 'A'

So you've obtained an entangled pair, AB, out of some black box we'll call action 1.

Your second act of entanglement can, theoretically, be an entirely unrelated property such that we might as well be talking about particles C & D, but I'm ignoring that possibility.

The point is that this second act of entanglement is itself a measurement and Action: you've done something that determines the quantum state of A and collapses the wave function formed in action 1. Then you entangling A and B again.

So you're back where you started, with A and B having an umeasured property which will, when A is measured, effect a spooky action at a distance that constrains B. And you don't have any information that links the second measurement to the first one: the values are independant.

Of course, you might commit Ation 2 in such a way that the result isn't 'measured' except that you know it has *changed* AB. Let's say you know it's reversed the polarisation on both. So the wave function does not collapse, and the particles remain entangled.

Clever, except that you didn't know what the value was in the first place, and you obviously still don't know now, until you measure the polarisation at A. Subsequent measurements of B will, by spooky action at a distance, be the predictable counterpart.

So you've achieved nothing.