Artur Ekert

A physicist who, as a doctoral student at Oxford, was struck by a connection between Bell's theorem and secret keys.

In 1991 he proposed a protocol (E91) in which two people share entangled pairs and test them with Bell's inequality. If the test is passed, no eavesdropper can have known the outcomes in advance, and the results give a secure key. Security rests on the laws of physics themselves rather than on the assumption that an attacker has limited computing power.

E91 tied the foundation question of whether the world is locally real to a practical technology. It also leads to device-independent cryptography, in which even a distrusted device can be checked by the statistics it produces.