The fact that the state of Bob’s photon changes instantaneously when Alice makes her Bell-state measurement is often confused by nonphysicists as a technology that could be used for communicating faster than light. The error is in thinking that an entangled photon encodes the information, when in fact the information is encoded in the correlation between the entangled photons.
Look back at Figure 152. Without the results of Alice’s Bell-state measurement, Bob wouldn’t know which one of the four corrections to apply to his photon to replicate. x Since these are completely random, Bob’s attempt at guessing which correction to apply to every photon would succeed only 25% of the time, and he won’t even know when he guessed correctly. His data stream would be as useless as if he had taken a beam of photons with completely random polarizations. Bob must simply wait for the classical message from Alice to properly extract any useful information, since the information is encoded in the correlation between Alice’s and his measurements. Einstein’s Relativity is not violated by the quantum teleporter, because even with “spooky action at a distance” instantaneously connecting the entangled photons, no information can be conveyed any faster than it takes for Alice’s classical message to arrive to Bob.
Few people outside the physics community understand why superluminal communication is so problematic. After all, no major issues seem to be caused in Star Trek by the use of “subspace communications” to establish nearly instantaneous contact with people and places light-years away. However, being able to transfer information at speeds faster than light opens up the possibility of communicating with the past, which creates paradoxes that give real physicists real heartburn.
Imagine that Alice is at rest and Bob is traveling in a spaceship close to the speed of light c, such that Alice’s clock appears to run twice as fast as Bob’s.¶¶¶ If Alice could send Bob a signal that traveled faster than c, it would appear to Bob that her signal was going backward in time. When Bob replied to Alice, his signal would reach Alice before she sent the original message. Thus, Alice could use Bob as a relay to send a message into her own past.
Any mechanism that would allow one to build a device capable of signaling to the past (commonly known as a “time telegraph”) is usually taken by physicists as evidence that it is impossible, because it violates the order of cause and effect. For example, when physicist Nick Herbert proposed a quantum cloning machine that could copy a single photon exactly,60 Israeli physicist Asher Peres prompted the physics community to look for the flaw in this proposal, since it would lead to the time telegraph paradox, indicating something had to be wrong in principle with this idea.
The argument is that if Bob had a quantum cloning machine, he could take the photon from the entangled pair that he shared with Alice and make some large number of perfect copies. He would then take all of his cloned photons (say 100 of them) and pass them through a vertical polarizer. Here comes the clincher: if Alice measures her photon with a vertical or a horizontal polarizer, either 0% or 100% of Bob’s photons will go through the polarizer. However, if Alice measured her photon with a 45° polarizer, only 50% of his photons will make it through his polarizer. Therefore, Alice could instantaneously signal Bob by simply changing the angle of her polarizer.
In fact, within a few months, physicists Wojciech Zurek, William Wootters, and Dennis Dieks proved mathematically that it is impossible to clone quantum information, leading to the so-called Quantum No-Cloning Theorem.61,62 Interestingly, the No-Cloning Theorem in itself doesn’t prevent superluminal communication via quantum entanglement, but it does prevent it through the mechanism proposed by Nick Herbert, which had seemed promising.
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