Quantum key distribution (QKD) allows two distant parties to share secret keys with the
proven security even in the presence of an eavesdropper with unbounded computational
power. Recently, GHz-clock decoy QKD systems have been realized by employing ultrafast
optical communication devices. However, security loopholes of high-speed systems have
not been fully explored yet. Here we point out a security loophole at the transmitter of the
GHz-clock QKD, which is a common problem in high-speed QKD systems using practical …