In quantum communication, a secret key is transmitted using single photons. Eavesdropping on such a channel is supposedly impossible: any interception attempt disturbs the particle and leaves a trace. But the idea has a weak spot — the real hardware, which sometimes behaves differently than theory dictates.
A single-photon detector is like a doorbell that goes off not only when you press the button but also when someone knocks loudly on the wall. The researchers tested an avalanche photodetector operating at a frequency of 312.5 megahertz. They found that as the pulse energy increases, it clicks not only more often but also noticeably earlier — the shift exceeds 2 nanoseconds. For comparison, light travels only 60 centimeters in that time.
This delay opens a loophole. An attacker can tune the brightness of a flash so that the detector's click jumps from one time slot to the next. Then the received bit will not be the one sent by the legitimate user, and the security proofs no longer hold. The authors proposed two attack schemes that exploit this effect.
Quantum security remains solid on paper, but each new look at the hardware brings us back to a simple thought: security is not just mathematics, it's also soldering.
The shift in the detector's firing time exceeds 2 nanoseconds — in that time, light travels only 60 centimeters.