Quantum key distribution (QKD) uses the principles of quantum mechanics to establish shared cryptographic keys between two parties with information-theoretic security — meaning the security is guaranteed by the laws of physics rather than the assumed computational difficulty of mathematical problems. The BB84 protocol (Bennett and Brassard, 1984), the first QKD protocol, encodes key bits in the polarization states of individual photons and exploits the no-cloning theorem: any eavesdropper attempting to intercept and copy the photons inevitably introduces detectable disturbances.
Commercial QKD systems are available from companies including ID Quantique (Switzerland), Toshiba (Japan/UK), and several Chinese companies. These systems typically operate over fiber optic links at distances up to 100-400 km, limited by photon loss in optical fiber. Key rates range from kilobits to megabits per second depending on distance and detector technology. China has deployed the world's largest QKD network, the Beijing-Shanghai backbone spanning 2,000 km using trusted relay nodes, and demonstrated satellite-based QKD over 1,200 km using the Micius satellite.
QKD faces practical challenges: it requires dedicated optical infrastructure (cannot share fibers easily with classical telecom traffic due to noise), has limited range without quantum repeaters (which are still experimental), and protects only the key exchange — not the subsequent encrypted communication, which relies on classical symmetric ciphers. Critics argue that post-quantum cryptographic algorithms (like the NIST-standardized lattice-based schemes) provide adequate security against quantum computers at far lower infrastructure cost. Proponents counter that QKD provides unconditional security against any future attack, including those using mathematical breakthroughs or unforeseen quantum algorithms.