Phys. Rev. Lett. 112, 070503 (2014)

In recent years, the security of cryptography schemes based on quantum key distribution (QKD) has come under scrutiny due to the presence of experimental loopholes and innovative hacking approaches. The latest threat to such systems has now been outlined by Audun Bugge and co-workers based in Norway, Sweden, Malaysia and Canada. The team propose a different type of quantum hacking attack on QKD systems in which an eavesdropper actively engineers new loopholes by using damaging laser illumination to permanently change the properties of the system components. The scientists investigated widely used silicon avalanche photodiodes (APDs), which are employed as single-photon detectors in several QKD experiments. A high-power continuous-wave 807-nm laser beam was focused to a 50-μm-diameter spot on an APD. The detector dark count rate and photon detection efficiency were measured as a function of the illumination power. After laser irradiation at a power of about 1.5 W, the APDs developed a large dark current. This blinds the passively quenched detector, reducing the photon detection efficiency and the dark count rate to zero. The laser damage is permanent and renders APDs completely insecure for QKD applications.