Definition: quantum cryptography

An encryption method that can detect eavesdropping. Using optical transmission to send a secret key to the other side, quantum cryptography draws on the inherent properties of photons, which become slightly altered if they are observed by an intruder. When an alteration is detected, the receiver knows the sender's key has been compromised. See cryptography, quantum secure and quantum computing.

Polarized and Entangled Photons
One method relies on the polarization of the photons, which will be altered if observed (see QKD). Another method uses photon pairs that exhibit a correlation between them. Any observation along the way weakens the correlation, which can be detected.