  A crucial goal for quantum key distribution (QKD) is to transmit
unconditionally secure keys over long distances. Previous studies show that the
key rate of point-to-point QKD is limited by a secret key rate capacity bound,
and higher key rates would require quantum repeaters. In 2018, the seminal
twin-field (TF) QKD protocol was proposed to provide a remarkable solution to
overcoming the linear secret key capacity bound. This article presents an
up-to-date survey on recent developments in this area, including the security
proofs of phase-matching QKD and other TF-QKD type protocols, the theoretical
examinations of these protocols under realistic conditions, and the recent
experimental demonstrations.
