Quantum Cryptography (BB84)
Bennett and Brassard's 1984 protocol lets two people agree on a secret key using single particles of light; a spy who measures them disturbs them and is noticed.
Open in the interactive tree →Charles Bennett and Gilles Brassard published the scheme in 1984, Stephen Wiesner had written the first paper on quantum cryptography around 1970, but it was published only in 1983. The sender encodes random bits in one of two light-polarisation settings; after the receiver measures, both compare their settings in public and keep only the matching ones, and a high error rate in a test sample reveals eavesdropping. The first working prototype, built at IBM in 1989, sent its quantum signals over 32 cm of free air; by 2000 the record over optical fibre was about 50 km. This protocol is the starting point of today's quantum key distribution.
Prerequisites
- Quantum Mechanics1925Measuring a quantum state disturbs it, which exposes a spy.
- Information Theory1948Error checks and shortening the key rely on information theory.
Unlocks
- Quantum Key Distribution2016Quantum key distribution by satellite and fibre is built on the BB84 protocol and its variants.