Quantum cryptography was designed to provide a new approach to the problem of distributing keys for private-key
cryptography. The principal idea is that security can be ensured by exploiting the laws of quantum physics and, in
particular, by the fact that any attempt to measure a quantum state will change it uncontrollably. This change can be
detected by the legitimate users of the communication channel and so reveal to them the presence of an eavesdropper. In this paper I explain (briefly) how quantum key distribution works and some of the progress that has been made towards making this a viable technology. With the principles of quantum communication and quantum key distribution firmly established, it is perhaps time to consider how efficient it can be made. It is interesting to ask, in particular, how many bits of information might reasonably be encoded securely on each photon. The use of photons entangled in their time of arrival might make it possible to achieve data rates in excess of 10 bits per photon.