The sending-or-not-sending twin-field (SNS-TF) protocol has so far proved to be a highly promising strategy for achieving high rates over long distances in quantum key distribution (QKD) applications. In fact, by tolerating large misalignment errors, this protocol can surpass the repeaterless bound in more effective ways, which is a crucial factor in the realization of long-distance QKD.
Source:
https://phys.org/news/2020-03-fiber-qkd-transmission-distance-km.html