Inducing a detector efficiency mismatch to hack a commercial quantum key distribution system

Quantum key distribution (QKD) is poised to be the first widespread implementation of quantum communication. In principle, it offers unconditional security: an eavesdropper introduces errors and thus cannot remain concealed from the legitimate parties. However, in practical implementations the actual security depends on a host of technological and protocol-operational components. We propose and experimentally demonstrate a hack to induce a large and deterministic temporal efficiency mismatch between the two detectors of Clavis2, a commercial QKD system from ID Quantique.