Quantum Cryptography

Quantum Cryptography was born in the early seventies when Stephen Wiesner wrote “Conjugate Coding”, which unfortunately took more than ten years to see the light of print [50]. In the mean time, Charles H. Bennett (who knew of Wiesner’s idea) and Gilles Brassard picked up the subject and brought it to fruition in a series of papers that culminated with the demonstration of an experimental prototype that established the technological feasibility of the concept [5]. Quantum cryptographic systems take advantage of Heisenberg’s uncertainty relations, according to which measuring a quantum system, in general, disturbs it and yields incomplete information about its state before the measurement. Eavesdropping on a quantum communication channel therefore causes an unavoidable disturbance, alerting the legitimate users. This yields a cryptographic system for the distribution of a secret random key between two parties initially sharing no secret information (however they must be able to authenticate messages) that is secure against an eavesdropper having at her disposal unlimited computing power. Once this secret key is established, it can be used together with classical cryptographic techniques such as the Vernam cipher (one-time pad) to allow the parties to communicate meaningful information in absolute secrecy. Quantum cryptography is best known for key distribution [7]. A short summary of this so-called BB84 protocol is provided in Section 1.2. A remarkable surge of interest in the international scientific and industrial community has propelled quantum cryptography into mainstream computer science and physics. Furthermore, quantum cryptography is becoming increasingly practical at a fast pace. The first quantum key distribution prototype, built in 1989, worked over a distance of 32 centimetres [5], [11]. Since then, many additional experimental demonstrations have been set up, covering distances of tens of kilometres. Consult [46] or [42] for popular accounts of the state of the art in experimental quantum cryptography.