I read occasionally popular science articles and from time to time encounter issues about quantum information teleportation. (this one for example http://www.physorg.com/news193551675.html)
So far I have the following understanding of basic principles:
- after measurement entangled particles have identical random state (i.e. we can not predict the result, but we know that it will be similar for both particles).
- "decoding key" should be sent by traditional channel
As I understand the underlying process is the following:
- Some amount of entangled particles is sent to A and B.
- Particles are measured at A and B and results R (equal in both places) are recorded.
- Knowing the results A computes a "decoding key" (some function F, that F(R) has some valuable meaning) which is sent by traditional means to B.
- B applies key to it's copy of R, and thus receives information.
The benefit is that amount of information that needs to be transmitted by traditional means is small.
In case everything stated before is correct, I have the question: how is this "quantum information teleportation" process differs from just sending two identical sets of random data to both destinations by traditional means?
I could be wrong about understanding of principles of quantum information teleportation, in this case, please, correct me.
Best Answer
You're slightly mistaken about the mechanism for teleportation. The process does involve sharing an entangled pair of particles, but the individual states of those particles are never measured. Instead, a joint measurement is made of the state of particle whose state A wants to teleport to B and one of the two entangled particles-- basically, asking "are you in the same state, or different states?" Based on the outcome of that measurement, which is sent to B via a classical channel, B does one of a small number of simple operations to the other entangled particle, which leaves it in exactly the state of the original particle that was being teleported.
The advantage of this process is that the state of A's original particle is never directly measured, which means that it arrives at B completely intact. This is the only sure way to transmit a single unknown quantum state without sending the particle in that state directly. You might think you could just make a copy of the state, and send the copy, but that is forbidden by the "no-cloning" theorem-- it is impossible to make an exact duplicate of a single quantum state unless you know in advance what that state is.
The key to the process is that the entangled pair of particles do not have well-defined states, but are correlated in a non-local fashion. You're not sending a set of random numbers with definite values from one place to another, you're sending an indeterminate state whose exact value will not be determined until it is measured, at which point its value will absolutely determine the state of a second particle a long distance away.
The process is of interest to people studying quantum information because it may be the surest way to move quantum information from one place to another. It's not going to allow you to build a Star Trek transporter any time soon, but it might be useful for building a quantum computer, or for connecting together quantum computers at distant locations.