BB84 Quantum Key Distribution

BB84 (Bennett & Brassard 1984): the first quantum key distribution protocol. Alice sends qubits in one of two bases: rectilinear {|0⟩,|1⟩} or diagonal {|+⟩,|−⟩}. Bob measures in a randomly chosen basis. After transmission, they publicly announce bases (not bits) and keep only bits where their bases agreed — the "sifted key."

If Eve intercepts and remeasures each qubit (intercept-resend attack), she induces a 25% quantum bit error rate (QBER) on the sifted key — noise that wasn't there. Alice and Bob estimate the QBER on a sample; above the security threshold (~11% for BB84), they abort. The no-cloning theorem (Wootters-Zurek 1982) prevents Eve from copying unknown quantum states without disturbance — making QKD information-theoretically secure.