The E91 Protocol, Entanglement-Based Quantum Key Distribution

E91 is the protocol that taught quantum cryptography to draw its security from entanglement itself. Proposed by Artur Ekert in 1991, it offered a strikingly different route to a shared secret key from the one Bennett and Brassard had taken seven years earlier. Where the earlier approach leaned on the impossibility of copying a quantum state, E91 grounds its safety in the strange correlations of entangled particles and the famous theorem of John Bell. This guide explains what the E91 protocol is, how it builds a key out of entanglement, and why a result from the foundations of physics turns out to guarantee its security. It also sets E91 beside its older cousin and looks at how the idea has grown into some of the most ambitious schemes in modern quantum communication. By 1991 quantum key distribution already existed, thanks to the BB84 protocol, in which one party prepares photons and sends them to another. Artur Ekert, then a doctoral student at Oxford, asked whether the same goal could be reached from a completely different starting point, using pairs of entangled particles shared between the two parties. The answer reshaped how physicists thought about…