Quantum Key Distribution (UA)

Quantum Key Distribution (QKD) is a secure communication method that leverages the principles of quantum mechanics to generate and distribute cryptographic keys. Unlike classical key distribution methods that rely on computational complexity for security, QKD's security is based on the fundamental laws of physics. The most common protocols, such as BB84 and E91, involve encoding bits of a secret key into quantum states of photons, typically their polarization. For instance, in the BB84 protocol, Alice sends photons to Bob, randomly choosing between two bases (e.g., rectilinear and diagonal) to encode each bit. Bob also randomly chooses a basis to measure each incoming photon. After the transmission, Alice and Bob publicly compare the bases they used for each photon. They discard measurements where their bases did not match, as these are inherently random. The remaining measurements, where bases matched, should ideally correlate perfectly to form a shared secret key. The security of QKD arises from the fact that any attempt by an eavesdropper (Eve) to intercept or measure the quantum states will inevitably disturb them. This disturbance, such as a change in photon polarization, can be detected by Alice and Bob by checking a subset of their correlated bits for errors. If the error rate exceeds a certain threshold, they can infer the presence of an eavesdropper and discard the compromised key. QKD systems typically consist of a quantum channel (e.g., optical fiber or free-space link) for transmitting quantum states and a classical channel for public communication (e.g., basis reconciliation and error correction). Trade-offs include distance limitations due to photon loss and decoherence, the need for specialized hardware, and the fact that QKD only distributes keys, not encrypts data itself; it must be paired with a classical symmetric [encryption](/uk/terms/symmetric-encryption) algorithm like AES.