to enable them to communicate in absolute privacy.33 In particular, the laws of quantum mechanics allow two particles (such as photons of light in a fiber-optic cable) to be put in a state of ''entangled" information. In such a state, any measurement of one of the particles necessarily disturbs the entanglement. Thus, eavesdropping on a quantum channel used to communicate a key will inevitably be detected by the intended recipient of the key, at which point a new key can be transmitted.
A working quantum cryptography apparatus has been developed, although the sending and receiving mechanisms are only 30 centimeters apart. The creators of this apparatus34 believe that nothing in principle limits the technique from being used over much greater distances. At the same time, they note that quantum key distribution must compete with classical techniques for key exchange, which are much cheaper over long distances.
33 The description in this subsection is taken from Charles Bennett et al., "Quantum Cryptography," Scientific American, Volume 267(4), October 1992, pp. 50-57.
34 Bennett et al., "Quantum Cryptography," 1992.