A cryptographic technique that uses quantum properties (typically photon polarization) to share a secret key between two parties with the guarantee that any eavesdropping attempt will be detected — provides information-theoretic security from the laws of physics.
QKD's most famous protocol is BB84 (1984). The sender encodes random bits in randomly chosen photon polarization bases; the receiver measures in randomly chosen bases. They publicly compare which bases they used and keep only the bits where they happened to match. Eavesdropping disturbs the quantum states detectably, so any interception leaves evidence. QKD is deployed in some banking, government, and telecom networks today (often via dedicated fiber), but is distinct from 'post-quantum cryptography' which is classical algorithms resistant to quantum attacks.
Using QKD over a dedicated fiber link between two bank data centers to exchange AES-256 keys, with eavesdropping detection from quantum measurement disturbance.
QKD is the only cryptographic technique with information-theoretic security from physics — but its deployment requires specialized hardware and is not a drop-in replacement for everyday encryption.
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