Security of practical modulator-free quantum key distribution

Abstract Recent advancements in quantum key distribution have led to the development of various modulator-free transmitters. Among their advantages, they offer enhanced security against Trojan-horse attacks. However, practical implementations emit residual pulses that, while not used in the quantum communication, still carry information about Alice’s state preparation process. While the intensity of these pulses is typically attenuated, the extinction ratio of practical intensity modulators is always finite, and therefore it remains crucial to account for the residual information leakage at the security-proof level. In this work, we prove the security of these transmitters in such setting and evaluate their performance. We find that the secret-key rate of the protocol is severely affected when the information leakage is not sufficiently small, which highlights the importance of taking into account this imperfection.