  Measurement-device-independent quantum key distribution (MDI-QKD) can
eliminate all detector side channels and it is practical with current
technology. Previous implementations of MDI-QKD all use two symmetric channels
with similar losses. However, the secret key rate is severely limited when
different channels have different losses. Here we report the results of the
first high-rate MDI-QKD experiment over $asymmetric$ channels. By using the
recent 7-intensity optimization approach, we demonstrate $>$10x higher key rate
than previous best-known protocols for MDI-QKD in the situation of large
channel asymmetry, and extend the secure transmission distance by more than
20-50 km in standard telecom fiber. The results have moved MDI-QKD towards
widespread applications in practical network settings, where the channel losses
are asymmetric and user nodes could be dynamically added or deleted.
