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Abstract : |
In general, observing a quantum system disturbs it, and prevents the observer from learning its exact state before the observation. Therefore, if quantum systems are used to carry information, an eavesdropper, or even the intended recipient, may be prevented from getting out all the information that the sender put in. This negative feature of quantum mechanics has recently been put to positive use in the arts of private and discreet communication. The goal of privacy is illustrated by two persons wishing to communicate over a public channel such as newspaper ads or electronic mail, via coded messages no one else can understand. The subtler goal of discretion is illustrated by two singles exploring the possibility of a date, who wish to communicate in a way that protects them, not from eavesdroppers, but from each other. For example, if Alice likes Bob, Bob should only be able to learn this fact if he also likes Alice, and vice versa. Or Alice and Bob might seek to make some more complicated decision based on private information, while protecting its confidentiality as much as possible. Both privacy and discretion can be achieved by using quantum uncertainty to do what mathematics alone cannot do. Mathematics offers imperfect solutions to these problems, in in the form of public key cryptography (PKC)[1]. Before the introduction of PKC in 1976, private communication over a public channel was thought to be impossible unless the two users had agreed beforehand on some random secret information that no one else knew. This information would serve as a cryptographic key, enabling Alice to scramble her messages in a way only Bob, knowing the same key, could unscramble. Privacy thus appeared to be generally unavailable to users lacking the means and foresight by which diplomats and spies share secret keys with their intended correspondents. Classical cryptography offered no purely mathematical solution to this "key distribution problem ". Meanwhile the only solution to the discreet communications problem appeared to be for Alice and Bob to confide their secrets in a trusted intermediary, who might later choose to blackmail them., |
