  We developed a countermeasure against blinding attacks on low-noise detectors
with a background noise cancellation scheme in quantum key distribution (QKD)
systems. Background noise cancellation includes self-differencing and balanced
avalanche photon diode (APD) schemes and is considered a promising solution for
low-noise APDs, which are critical components in high-performance QKD systems.
However, its vulnerability to blinding attacks has been recently reported. In
this work, we propose a new countermeasure that prevents this potential
security loophole from being used in detector blinding attacks. An experimental
QKD setup is implemented and various tests are conducted to verify the
feasibility and performance of the proposed method. The obtained measurement
results show that the proposed scheme successfully detects occurring
blinding-attack-based hacking attempts.
