QKD

Quantum key distribution (QKD) is a method for distributing cryptographic keys between two parties with security guaranteed by the laws of quantum mechanics. It uses quantum states of light or other quantum systems to encode key bits, and relies on the fact that observation generally disturbs quantum states and that quantum information cannot be copied perfectly. QKD requires a quantum channel and an authenticated classical channel for public discussion.

The most studied protocols are BB84, proposed in 1984 by Bennett and Brassard, and Ekert's E91, which uses entangled particles. In BB84, bits are encoded in non-orthogonal bases and Bob’s measurements succeed with a basis match; mismatches are discarded. E91 relies on entanglement and Bell inequality correlations to detect eavesdropping. Practical implementations often use decoy-state methods to defend against photon-number-splitting attacks.

After the quantum transmission, the parties perform sifting to keep only the matching basis results, followed by error correction and privacy amplification to produce a shorter but provably secure key. An eavesdropper attempting to gain information introduces disturbances that raise the quantum bit error rate, enabling detection and exclusion of compromised data.

Field deployments exploit optical fiber, free-space, and satellite links. Realistic security depends on device models; imperfections and side-channel attacks are active research areas. Techniques such as device-independent QKD and measurement-device-independent QKD address some trust assumptions. Ongoing work also covers finite-key security, higher key rates, and integration with conventional networks.

QKD does not replace encryption but provides a secure way to distribute keys for symmetric ciphers. It has progressed from laboratory demonstrations to commercial systems and standards efforts, though widespread adoption is influenced by cost, infrastructure needs, and the development of robust, realistic security proofs.