  The performance of quantum key distribution (QKD) is severely limited by
multiphoton emissions, due to the photon-number-splitting attack. The most
efficient solution, the decoy-state method, requires that the phases of all
transmitted pulses are independent and uniformly random. In practice, however,
these phases are often correlated, especially in high-speed systems, which
opens a security loophole. Here, we address this pressing problem by providing
a security proof for decoy-state QKD with correlated phases that offers key
rates close to the ideal scenario. Our work paves the way towards
high-performance secure QKD with practical laser sources, and may have
applications beyond QKD.
