  Many quantum key distribution systems employ a laser followed by an optical
attenuator to prepare weak coherent states in the source. Their mean photon
number must be pre-calibrated to guarantee the security of key distribution.
Here we experimentally show that this calibration can be broken with a
high-power laser attack. We have tested four fiber-optic attenuator types used
in quantum key distribution systems, and found that two of them exhibit a
permanent decrease in attenuation after laser damage. This results in higher
mean photon numbers in the prepared states and may allow an eavesdropper to
compromise the key.
