National University of Singapore researchers have shown that “uncrackable” quantum cryptographic systems can in fact be cracked by using lasers to “blind” photon polarization detectors and force them to detect fake quantum-entanglement correlations, falsely indicating that a key is certified as secure.

Quantum cryptography — which uses the quantum states of photons to encode information for transmission — exploits the fact that measurements cannot be made of a quantum system without disturbing it. It was thought impossible for an eavesdropper to intercept a quantum encryption key without disrupting it and triggering alarm bells.