<- Virtual Exhibitions in Informatics

Quantum Cryptography

The roots of Quantum Cryptography date back to the late 1960s, when Stephen Wiesner published his idea of so called 'Quantum Money'.


Charles Bennett

No one did show any interest in Wiesner’s idea except for Charles Bennett, who had been an undergraduate at the same university. Although Bennett anticipated the potential of Wiesner’s manuscript, he could not immediately find any practical approach to the concept. Nevertheless he did not stop to reread the paper in the following years and still fascinated with Wiesner’s outstanding idea he began to discuss the whole topic with a computer scientist from Montreal University named Gilles Brassard in the early eighties.

The crucial base of the Quantum Money was the fact that it was impossible to accurately perceive the  polarization of the trapped photons without knowing which filter to use and Bennett and Brassard discovered that the same effect could be used in cryptography to transmit an enciphered message as a stream of photons.
They were trying to develop a protocol to transmit encrypted data between two participants named Alice and Bob, without giving a third person in the middle named Eve any chance to decrypt the message sent. It was obvious that Eve would have to face the unsolvable problems of not knowing which polarization filters to use and moreover getting in danger of revealing her presence by modifying several photons when applying the wrong filters.
But what seemed to be the weakness of the whole concept was the fact, that Bob wouldn’t know the correct sequence of filters either. That would have leaded to the well known problem of key exchange and the whole concept would not have been anything revolutionary new except the use of light as the medium for the transmission.

After some years of research on the topic and thinking twice of each detail of such a secure transmission of data the whole concept seemed to be a flop but then in 1984 Bennett and Brassard were discussing the whole course of events again in a very informal atmosphere while standing at a station and waiting for the train which should take Brassard back to Montreal. In these few minutes they invented the protocol today known as ‘BB84’, which solves the problem of key exchange and provides secure communication on the basis of transmitting photons. If Brassard’s train would have arrived in time this brilliant concept maybe would not be known today.