  When elementary quantum systems, such as polarized photons, are used to
transmit digital information, the uncertainty principle gives rise to novel
cryptographic phenomena unachievable with traditional transmission media, e.g.
a communications channel on which it is impossible in principle to eavesdrop
without a high probability of disturbing the transmission in such a way as to
be detected. Such a quantum channel can be used in conjunction with ordinary
insecure classical channels to distribute random key information between two
users with the assurance that it remains unknown to anyone else, even when the
users share no secret information initially. We also present a protocol for
coin-tossing by exchange of quantum messages, which is secure against
traditional kinds of cheating, even by an opponent with unlimited computing
power, but ironically can be subverted by use of a still subtler quantum
phenomenon, the Einstein-Podolsky-Rosen paradox.
