# Quantum teleportation and no-communication theorem

According to the Wikipedia article for the No-communication theorem:

In very rough terms, the theorem describes a situation that is analogous to two people, each with a radio receiver, listening to a common radio station: it is impossible for one of the listeners to use their radio receiver to send messages to the other listener. This analogy is imprecise, because quantum entanglement suggests that perhaps a message could have been conveyed; the theorem replies 'no, this is not possible'.

According to the Jet Propulsion's recent article on quantum teleportation: http://www.jpl.nasa.gov/news/news.php?feature=4384

they can effect an entangled photon (B) with another photon (A) to change the state of the other entangled photon.

Doesn't this contradict what the no-communication theorem states ?

• No. Entanglement only produces correlations between the two ends of a quantum communication link. Without an explicit second communication channel (limited to c) that correlation can not be converted into actual information. Commented Dec 31, 2014 at 11:00
• I'm a little bit confused of why one can't extract information from the correlation between the two entangled particles. Can you please elaborate, give an example or tell me what topic I should read more about on my own. Commented Dec 31, 2014 at 11:15
• Because correlation is not causation. An entanglement experiment basically gives you two random streams of data. There is no information in either. The information can only be recovered by combining both, but since it takes a conventional channel to get them into the same place, the recovery of information from the correlation of the entangled quantum system can only happen at he speed of light. Commented Dec 31, 2014 at 12:13
• @D8F1F488 - Basically, communication is impossible because if experimenter #1 performs a given measurement after experimenter #2 has already performed a measurement on another part of the same entangled system, the total probability that #1 will get a certain result (as opposed to the conditional probability he'll get a certain result given knowledge of what experimenter #2 measured) is totally unaffected by what measurement #2 performed, or what outcome #2 observed. I tried to spell out this notion in detail here, if it helps. Commented Dec 31, 2014 at 15:20

## 1 Answer

Quantum teleportation requires a "classical channel" of information to be communicated between the two experimenters, so it doesn't violate the no-communication theorem because that theorem only rules out the possibility that two experimenters could communicate purely by their choice of measurements on parts of an entangled system. Referring to the schematic diagram of quantum teleportation below (from this page), the first experimenter performs a disruptive measurement on the system to be teleported (A) and also performs a measurement on one half (B) of a larger entangled system, then sends data on her measurements in some ordinary classical way (radio waves, an electrical cable, whatever--this is the "Send Data" arrow in the diagram) to the second experimenter, who then uses that data to perform just the right type of measurement on the other half of the entangled system (C) so that its state becomes identical to original state of A the moment before the disruptive measurement.