Quantum communication moves into the unknown By David Deutsch and Artur Ekert Information is physical and any processing of information is always performed by physical means - an innocent-sounding statement, but its consequences are anything but trivial, In the last few years there has been an explosion of theoretical and experimental innovations which, their discoverers claim, are creating a fundamental new discipline: a distinctively quantum theory of information. Quantum physics allows the construction of qualitatively new types of logic gates, absolutely secure cryptosystems (systems that combine communications and cryptography), the cramming of two bits of information into one physical bit and, as has just been proposed, a sort of "teleportation", Here we describe the last two "miracles". Classical information theory agrees with everyday intuition: if you want to send a message using an object which can be put into one of N distinguishable states, the maximum number of different messages that you can send is N . For example, a single photon can have only two distinguishable polarisation states, say "left-handed" and "right-handed". So if you send a message by preparing the polarisation of a single photon and transmitting it, it is obvious that you can send no more than two distinguishable messages, i.e, one bit of information. 0bvious, but false. | |
|
