  In principle, quantum key distribution (QKD) offers unconditional security
based on the laws of physics. In practice, flaws in the state preparation
undermine the security of QKD systems, as standard theoretical approaches to
deal with state preparation flaws are not loss-tolerant. An eavesdropper can
enhance and exploit such imperfections through quantum channel loss, thus
dramatically lowering the key generation rate. Crucially, the security analyses
of most existing QKD experiments are rather unrealistic as they typically
neglect this effect. Here, we propose a novel and general approach that makes
QKD loss-tolerant to state preparation flaws. Importantly, it suggests that the
state preparation process in QKD can be significantly less precise than
initially thought. Our method can widely apply to other quantum cryptographic
protocols.
