KentuckyFC writes "Physicists have developed a new kind of quantum cryptography that uses position measurements to guarantee the security of a message. The technique is based on triangulation. Alice uses several transmitters to send messages to Bob who returns them immediately at the speed of light. If the return arrives within a certain time period, Alice can be certain that Bob is where he says he is. Physicists proved a few years ago that when the messages are purely classical this method is not secure because Eve can use any number of receivers to work out where Bob is and then use this information to trick Alice. However, the same physicists have now proved that the quantum version of the same position-based scheme is perfectly secure, essentially because Eve cannot easily measure the value of any qubits in the message. Alice and Bob go on to use the qubits to exchange a cryptographic key, a one-time pad, that they use to encrypt a message. The beauty of the technique is that a message encrypted in this way can be read only by somebody at a specific location, something that governments, banks, and the military, not to mention everybody else, may find useful."