  The ultimate aim of quantum key distribution (QKD) is improving the
performance of transmission distance and key generation speed. Unfortunately,
it is believed to be limited by the secret-key capacity of quantum channel
without quantum repeater. Recently, a novel twin-field QKD (TFQKD) [Nature 557,
400 (2018)] is proposed to break through the limit, where the key rate is
proportional to the square-root of channel transmittance. Here, by using the
vacuum and one-photon state as a qubit, we show that the TF-QKD can be regarded
as a measurement-device-independent QKD (MDI-QKD) with single-photon Bell state
measurement. Therefore, the MDI property of TF-QKD can be understood clearly.
Importantly, the universal security proof theories can be directly used for the
TF-QKD, such as BB84, six-state and reference-frame-independent schemes.
Furthermore, we propose a feasible experimental scheme for the
proof-of-principle experimental demonstration.
