  Twin-field quantum key distribution can overcome the secret key capacity of
repeaterless quantum key distribution via single-photon interference. However,
to compensate for the channel fluctuations and lock the laser fluctuations, the
techniques of phase tracking and phase locking are indispensable in experiment,
which drastically increase experimental complexity and hinder free-space
realization. Inspired by the duality in entanglement, we herein present an
asynchronous measurement-device-independent quantum key distribution protocol
that can surpass the secret key capacity even without phase tracking and phase
locking. Leveraging the concept of time multiplexing, asynchronous two-photon
Bell-state measurement is realized by postmatching two interference detection
events. For a 1 GHz system, the new protocol reaches a transmission distance of
450 km without phase tracking. After further removing phase locking, our
protocol is still capable of breaking the capacity at 270 km. Intriguingly,
when using the same experimental techniques, our protocol has a higher key rate
than the phase-matching-type twin-field protocol. In the presence of imperfect
intensity modulation, it also has a significant advantage in terms of the
transmission distance over the sending-or-not-sending type twin-field protocol.
With high key rates and accessible technology, our work provides a promising
candidate for practical scalable quantum communication networks.
