What is a quantum key distribution (QKD), and how does it work?

Quantum Key Distribution (QKD) is a method for securely sharing cryptographic keys between two parties by leveraging principles of quantum mechanics. Unlike classical key exchange methods, which rely on mathematical complexity for security, QKD ensures security through the physical properties of quantum states. If an eavesdropper attempts to intercept the key, the quantum states of the particles used in the exchange are altered, alerting the legitimate parties to the breach. This makes QKD theoretically immune to computational attacks, including those from future quantum computers.

A common QKD protocol is BB84, developed in 1984. Here’s how it works: Alice (the sender) encodes random bits into photons using one of two polarization bases—rectilinear (vertical/horizontal) or diagonal (45° or 135°). She sends these photons to Bob (the receiver), who randomly selects a basis to measure each photon. After transmission, Alice and Bob publicly compare their choice of bases (but not the actual bits) and discard results where their bases didn’t match. The remaining bits form a shared secret key. To detect eavesdropping, they compare a subset of these bits. If discrepancies exceed a threshold (indicating measurement interference), they discard the key and restart. For example, if Eve intercepts a photon, her measurement forces it into a specific basis, introducing errors when Bob measures it later.

Practical QKD implementations face challenges. Photon loss in fiber-optic cables limits transmission distances (typically under 300 km without quantum repeaters). Systems often use attenuated lasers instead of true single-photon sources, which can introduce security loopholes. Companies like ID Quantique and Toshiba have deployed QKD in metropolitan networks, integrating it with classical protocols like AES for hybrid encryption. Developers should note that QKD only secures key exchange—not the data itself—and requires classical authentication channels to prevent man-in-the-middle attacks. While not a replacement for existing encryption, QKD offers a future-proof layer for high-security scenarios, such as government or financial communications, where long-term key protection is critical.