  We propose a class of attacks on quantum key distribution (QKD) systems where
an eavesdropper actively engineers new loopholes by using damaging laser
illumination to permanently change properties of system components. This can
turn a perfect QKD system into a completely insecure system. A
proof-of-principle experiment performed on an avalanche photodiode-based
detector shows that laser damage can be used to create loopholes. After about 1
W illumination, the detectors' dark count rate reduces 2 to 5 times,
permanently improving single-photon counting performance. After about 1.5 W,
the detectors switch permanently into the linear photodetection mode and become
completely insecure for QKD applications.
