  Measurement-device-independent quantum key distribution (MDI-QKD) is immune
to all security loopholes on detection. Previous experiments on MDI-QKD
required spatially separated signal lasers and complicated stabilization
systems. In this paper, we perform a proof-of-principle experimental
demonstration of plug-and-play MDI-QKD over an asymmetric channel setting with
a single signal laser, in which the whole system is automatically stabilized in
spectrum, polarization, arrival time and phase reference. Both the signal laser
and the single-photon detectors are in the possession of a common server. A
passive timing calibration technique is applied to ensure the precise and
stable overlap of signal pulses. The results pave the way for the realization
of a quantum network, in which the users only need the encoding devices.
