quantum key distribution (QKD)

Quantum key distribution is a way for two people — call them Alice and Bob — to create a shared secret key, with security that rests on physics rather than on a hard math problem. Alice sends a stream of single qubits, each prepared in one of a few possible states; Bob measures them; and afterward they compare notes over an ordinary public channel to distill a string of bits only they know. That key is then fed into normal encryption to scramble their actual messages.

The clever part is how they catch a spy. In quantum mechanics, you cannot measure an unknown qubit without disturbing it, and you cannot copy it perfectly (the no-cloning theorem). So an eavesdropper, Eve, who intercepts and measures the qubits inevitably leaves fingerprints: she introduces errors in the bits Alice and Bob share. By sacrificing a random sample of their key and checking the error rate, Alice and Bob can tell whether anyone was listening. If the errors are low enough, they keep the key; if not, they throw it away and try again. Eve cannot quietly steal the key — listening is detectable.

Be clear about what QKD does and does not do. It secures the exchange of a key, not the rest of your computing world, and in practice it needs a separately authenticated channel (otherwise Eve could impersonate Bob), plus real hardware over real distance, which limits range and rate. It is also a different thing from post-quantum cryptography: QKD uses quantum physics to share keys, while post-quantum cryptography is ordinary classical algorithms designed to resist attack by a future quantum computer. They solve overlapping problems by completely different means.