  Quantum key distribution (QKD) is an attractive technology for distributing
secret encryption keys between distant users. The decoy-state technique has
drastically improved its practicality and performance, and has been widely
adopted in commercial systems. However, conventional intensity modulators can
introduce security side channels in high speed QKD systems because of their
non-stationary working points for decoy-state generation. Here, we analyze the
transfer function of an in-phase/quadrature (IQ) modulator and reveal its
superiority for stable decoy-state generation, followed by an experimental
demonstration. Thanks to their convenient two-level modulation and inherent
high speed, IQ modulators are ideal for use in high-speed decoy-state QKD
systems.
