External magnetic effect for the security of practical quantum key distribution

Quantum key distribution (QKD) allows remote parties to share secret keys with proven security. To guarantee the security of practical QKD, the imperfections in realistic devices need to be characterized and considered in practical security analysis. Particularly, a standard QKD system normally uses optical isolator or optical circulator in the transmitter to prevent the injection from external light. Here we find that the optical isolators and circulators, commonly based on the Faraday effect, are subject to the perturbation from external magnetic fields. With this, we provide a comprehensive analysis of the security for practical QKD due to external magnetic effect. We experimentally demonstrate the influence of magnetic fields on both the transmittance and the isolation of standard optical isolators and circulators. We analyze the security risks caused by the potential magnetic attacks, together with the corresponding countermeasures.