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Over The HorizonQuantum TeleportationIt won't let you beam up to the Enterprise or send a signal faster than the speed of light, but a new theoretical result making use of the Einstein-Podolsky-Rosen (EPR) effect does provide a surprising new way to transmit quantum information. The result is reported in a paper in a recent Physical Review Letters, by Charles H. Bennett, Gilles Brassard, Claude Crépeau, Richard Jozsa, Asher Peres and William K. Wootters. The paper builds on recent quantum cryptography work that uses quantum mechanics to transmit information in a form that cannot be surreptitiously eavesdropped upon. The latest work supposes that the sender, "Alice," has a particle "X" in an unknown quantum state and that she wants to convey that state to a receiver, "Bob," without resorting to simply taking particle X to him. Because of the uncertainty principle, in general Alice cannot measure the state of X with certainty and transmit a straightforward (classical) message to Bob. The new result provides a way for her to divide the information contained in the quantum state into quantum and classical parts and send them separately. The quantum information is transmitted by means of an EPR pair of particles -- two particles prepared in advance in a single pure quantum state and then widely separated. One is kept by Alice, one by Bob. Such an EPR pair has the unusual and much-debated property that results of measurements made by Alice and Bob on their respective particles will always be related by certain consistency requirements, even if Alice is on Earth and Bob at Alpha Centauri. Superficially this seems to imply that information passes between the two locations faster than the speed of light, but in fact it has been proven that the probabilistic nature of quantum measurements prevents the EPR effect from being used to transmit information faster than the speed of light. In the current scheme, Alice allows her half of the EPR pair to interact with her particle X. She then makes a measurement of the combined state of particle X and her EPR particle. This measurement has two consequences: it instantaneously alters the quantum state of Bob's EPR particle and it provides Alice with data that can be encoded into two bits of classical information (two zeros or ones). If Alice now transmits this information to Bob (for example, by an ordinary radio transmission), he can use the information to change his EPR particle into exactly the quantum state that particle X had originally. The importance of the scheme is probably not in any practical applications for transmitting quantum states, but rather in the light it sheds on quantum and classical information. In a sense, the process outlined divides the information contained in the quantum state into a classical part and a quantum part. The classical part consists of two bits of information that may be duplicated and broadcast by ordinary means. The quantum part can't be copied or broadcast to multiple locations, but it can be moved from one location to another, either by moving the particle itself, or by "quantum teleportation" using correlations such as the EPR scheme. Already the authors of the quantum teleportation paper are working on a definition and theory of quantum information. Graham Collins, NZSM, New York |
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