  Large-scale quantum communication networks are still a huge challenge due to
the rate-distance limit of quantum key distribution (QKD). Recently, twin-field
(TF) QKD has been proposed to overcome this limit. Here, we prove that
coherent-state-based TF-QKD is a time-reversed entanglement protocol, where the
entanglement generation is realized with entanglement swapping operation via an
entangled coherent state measurement. We propose a coherent-state-based TF-QKD
with optimal secret key rate under symmetric and asymmetric channels by using
coherent state and cat state coding. Furthermore, we show that our protocol can
be converted to all recent coherent-state-based TF-QKD protocols by using our
security proof. By using the entanglement purification with two-way classical
communication, we improve the transmission distance of all coherent-state-based
TF-QKD protocols.
