The first quantum key distribution protocol was BB84, introduced in 1984 by Charles Bennett and Gilles Brassard. Its name combines their initials with the year of publication, and it established the basic prepare-and-measure approach used in many later QKD systems.
In BB84, Alice encodes random bits in quantum states using one of two possible measurement bases and sends the resulting photons to Bob. Bob independently chooses bases at random. Afterward, they communicate over an authenticated classical channel to keep only the measurements made in matching bases, forming a shared key after error checking and privacy amplification.
The protocol’s security comes from a central quantum principle: non-orthogonal quantum states cannot be perfectly distinguished. An eavesdropper who measures photons without knowing the correct basis introduces detectable errors. The no-cloning theorem also prevents an interceptor from making a perfect copy of an unknown quantum state for later measurement.
BB84 is not the only QKD protocol. B92 followed in 1992 using two non-orthogonal states, while E91, proposed by Artur Ekert in 1991, uses entangled particles. Those are important successors, but BB84 is the canonical first protocol.