…Quantum entanglement means that two particles that become “entangled” through physical interaction will possess correlated states - if one is measured as possessing one value, the other can subsequently be measured as possessing the correlated value.
…Quantum teleportation means that if you produce an entangled pair of particles, you can input a photon to one to produce known information, which can be used to subsequently force its partner to produce an identical photon.
My questions are as follows:
(1) Does quantum teleportation work in reverse I.e. input two bits of known information to produce a photon possessing two qubits of information, then feed that photon into the particle’s partner to produce the same two bits of known information?
(2) I’m guessing that in between the two steps - when only one particle has been measured/interacted with - the entanglement is broken. Does completing the second step re-entangle the two particles? In other words, how hard is it to “reset” the particle pair so that it may teleport another two qubits of information?
(3) Why can’t this just be hidden variable, where the two entangle particles have been set to corresponding values A and A` by their interaction? Yes, I’ve heard of Bell’s theorum. I’m asking what distinguishes Bell’s theorum from an appeal to authority*.
the evidence suggests A
Bell’s theorum states not-A
therefore, not-A
I’m not sure I really get you here. What quantum teleportation allows you to do is to recreate the qubit that is to be teleported, I’m not sure what the reverse of that would be.
No,once a measurement is made the entanglement is destroyed
Quantum mechanics states that the correlation varies as the cosine of the angles of the measurements whereas a local hidden variables theorum of the kind ruled out by Bell states that the correlation varies linearly with the angle of the measurements.
All quantum teleportation really allows you to do is t circumvent the no cloning theorum of quantum mechanics. The acheivement of recreating the state of a particle at another location is nothing special in classical physics, quantum teleportation just allows you to get around a problem created by quantum physics.
(1) There is no possible experiment that can tell the difference between a local hidden variable theory and standard quantum mechanics.
(2) Bell: Yes there is. Here’s one.
(3) Oh. I guess you’re right. Let’s do that experiment a bunch and see what happens. Oh look, it comes out in favour of standard quantum mechanics.
(4) Yeah, but what about <insert effect of possible experimental error here> – maybe we can still have a hidden variable theory that is not eliminated because of that effect.
(5) Ok, let’s do a new experiment that eliminates that effect.
Repeat (4) and (5) ad Nauseam. No argument from authority there.
If you have two entangled particles, A and B, you can input a photon to A to produce classical information, which can be used to make B emit a photon that is identical to the original. I was asking what happens if you have two entangled particles, B and C, if you input classical information to B to make it emit a photon, and direct that photon into C.
I think you may be slightly confused AFAIK basic quantum teleportation consists of 3 particles of the same type A, B and C. If we say that B and C start off entangled and that we want to teleport the qubit contained in A from Alice who has particles A and B in her posession to Bob who has particle C in his possession then the end result is that particle C will contain the qubit that was intially contained in A.
I’m not sure I get the OP’s questions… Let’s perhaps have a look at how quantum teleportation works. There are two parties, the famous physicists Alice and Bob. Alice has in her possession a quantum state (for simplicity, a qubit) that she wants to transport to Bob, for some reason. Luckily, Alice and Bob already share an entangled pair – perhaps two electrons entangled wrt their spin. So, we have, as Pants above said, three quantum states: the one Alice wants to transmit, C, and the entangled pair Alice and Bob share, A and B. This quantum system thus is represented by a certain three-particle state.
Alice now makes a measurement on the two particles, A and C, in her possession, obtaining two bits of information. This measurement breaks the entanglement between A and B, and puts A and C into an entangled state. Bob’s particle will be in one of four possible states, all of which are related by a transformation to the state to be teleported. The two bits Alice obtained are now communicated to Bob; they tell him which of the four possible states his qubit is in, and accordingly, how to transform his state in order to make it identical to the one that was supposed to be transmitted. After this step is completed, Bob has a qubit in a state identical to he one C was in before the teleportation process – C’s state has thus been teleported.
In order to ‘reset’ the system, Alice and Bob first have to share an entangled pair again – you can’t create entanglement via local operations on distant quantum states.
No-cloning just says that there is no process that takes one (arbitrary) quantum state as an input, and outputs two ‘copies’ of the same state; this isn’t violated here, as after teleportation, C is no longer in the state that was teleported.
Yep, by circumvent I mean merely get around the problem posed by the no-cloning theorum to transferring the state of A to C. Obviously quantum teleportation is not at odds with the no-cloning theorum.
I agree with the other responders on the answers to (2) and (3). (1) hasn’t been addressed yet:
The answer is Yes; this is called superdense coding. The idea is that there is a basis of four maximally entangled (Bell) states for a pair of entangled qubits, and Alice can convert between these states using only operations local to the qubit she holds. So Alice performs one of four local unitary operations on her qubit and sends the qubit to Bob, who then can measure both qubits in the Bell basis to get two bits of information.
In case anyone’s interested, I’ve written up my thoughts on entanglement on my blog, with an emphasis of giving a somewhat intuitive account for the non-specialist. I went a little overboard, as I often do, so it’s a bit long perhaps, but this way, besides entanglement, you’ll also get holography! And string theory! And other buzzwords!