  Quantum key distribution (QKD) has been proved to be
information-theoretically secure in theory. Unfortunately, the imperfect
devices in practice compromise its security. Thus, to improve the security
property of practical QKD systems, a commonly used method is to patch the
loopholes in the existing QKD systems. However, in this work, we show an
adversary's capability of exploiting the imperfection of the patch itself to
bypass the patch. Specifically, we experimentally demonstrate that, in the
detector under test, the patch of photocurrent monitor against the detector
blinding attack can be defeated by the pulse illumination attack proposed in
this paper. We also analyze the secret key rate under the pulse illumination
attack, which theoretically confirmed that Eve can conduct the attack to learn
the secret key. This work indicates the importance of inspecting the security
loopholes in a detection unit to further understand their impacts on a QKD
system. The method of pulse illumination attack can be a general testing item
in the security evaluation standard of QKD.
