BB84 Protocol.

BB84, introduced by Charles Bennett and Gilles Brassard in 1984, is the canonical quantum key distribution (QKD) protocol. Alice transmits single photons polarized in one of four states drawn from two non-orthogonal bases — rectilinear (0°/90°) and diagonal (45°/135°) — each encoding a random bit. Bob measures each photon in a randomly chosen basis; over a public authenticated channel they then announce their bases and keep only the rounds where they matched (sifting). The security rests on the no-cloning theorem: any eavesdropper ("Eve") who measures an unknown photon in the wrong basis disturbs it, injecting errors.

Detecting the eavesdropper

Alice and Bob sacrifice a random subset of sifted bits to estimate the quantum bit error rate (QBER). For BB84, once QBER exceeds roughly 11%, no secret key can be distilled and the exchange is aborted. Below that, error correction plus privacy amplification compress the raw key into a shorter, information-theoretically secure key.

flowchart TD
  A[Alice: random bit + random basis] -->|polarized photon| B[Quantum channel]
  E[Eve intercepts?] -.measures & resends.-> B
  B --> C[Bob: measures in random basis]
  C --> D[Public channel: compare bases, sift]
  D --> F[Estimate QBER on sample]
  F -->|QBER > ~11%| G[Abort: eavesdropper likely]
  F -->|QBER low| H[Error correction + privacy amplification]
  H --> I[Shared secret key]

Real-world deployment

Practical systems use faint laser pulses rather than true single photons, exposing them to photon-number-splitting attacks; the decoy-state method (2003–2005) restores security. China's Micius satellite demonstrated satellite-to-ground decoy-state BB84 over 1,200 km in 2017 at ~1 kbit/s. Unlike lattice schemes such as CRYSTALS-Kyber, BB84 needs specialised optical hardware, so it complements rather than replaces post-quantum cryptography against "harvest now, decrypt later" threats.