Security flaws from time-varying active encoding in high-speed measurement-device-independent quantum key distribution

Quantum key distribution (QKD) can transmit secret keys with, in principle, information-theoretic security. However, bandwidth limitations in practical equipment threaten the security of high-speed (GHz) QKD systems. We propose and characterize a new side channel which arises when using active encoding. As an illustrative example, we focus on electro-optic phase modulation for polarization encoding at 1 GHz. We show that this side channel may reduce the maximum secure transmission distance by over 50% in a decoy state measurement-device-independent QKD protocol.