A topic in the Open Knowledge Graph — a free, open map of 15,290 topics and the order to learn them in.
Quantum key distribution (QKD) enables two parties to establish a shared secret key whose security is guaranteed by the laws of quantum mechanics, not computational hardness assumptions. The BB84 protocol (Bennett-Brassard 1984) works by encoding random bits in one of two conjugate bases (rectilinear or diagonal). An eavesdropper measuring the qubits inevitably disturbs them (by the no-cloning theorem and measurement disturbance), introducing detectable errors. After basis reconciliation and error estimation, the parties distill a secure key. BB84 achieves information-theoretic security — it is provably secure even against adversaries with unlimited computational power, including quantum computers.
Classical cryptography faces a fundamental problem: the security of widely used public-key systems (RSA, elliptic curves) rests on assumptions about the computational hardness of certain mathematical problems — assumptions that Shor's algorithm would break. Quantum key distribution offers a qualitatively different kind of security: one based on physics rather than mathematics. The BB84 protocol, proposed by Bennett and Brassard in 1984, uses quantum mechanics to distribute a shared secret key between two parties in a way that any eavesdropping attempt is detectable.
The protocol works as follows. Alice prepares random qubits, each encoding a random bit in a randomly chosen basis: either the Z basis (|0> for 0, |1> for 1) or the X basis (|+> for 0, |-> for 1). She sends these qubits to Bob, who measures each in a randomly chosen basis (Z or X). When their bases match (about 50% of the time), their bit values agree perfectly. When bases mismatch, Bob's result is completely random. Alice and Bob publicly compare their basis choices (not bit values) and keep only the rounds where they used the same basis — this is sifting, producing the raw key.
Eavesdropping detection comes next. Suppose Eve intercepts qubits, measures them, and resends them to Bob (an intercept-resend attack). Eve does not know Alice's basis, so she guesses randomly. When Eve guesses wrong (50% of the time), her measurement disturbs the qubit, and when Bob subsequently measures in Alice's correct basis, he gets a random result instead of Alice's bit. This introduces an error rate of approximately 25% in the sifted key. Alice and Bob sacrifice a random subset of their sifted key bits, compare them publicly, and check the error rate. An error rate significantly above the channel noise threshold indicates eavesdropping, and they abort.
If the error rate is acceptably low, Alice and Bob apply error correction (to fix the remaining errors) and privacy amplification (to eliminate any partial information Eve may have gained). The result is a shorter but provably secure shared secret key. The security proof, formalized by Lo, Chau, Shor, Preskill, and others, shows that any eavesdropping strategy — including sophisticated quantum attacks beyond intercept-resend — is detectable. The proof relies on the no-cloning theorem (Eve cannot copy the qubits and keep them for later analysis) and the information-disturbance tradeoff (gaining information about a quantum state necessarily disturbs it). QKD has been commercially deployed over fiber-optic links and demonstrated via satellite (the Micius experiment), making it the most mature application of quantum information science.