  Twin-Field quantum key distribution (TF-QKD) can beat the linear bound of
repeaterless QKD systems. After the proposal of the original protocol, multiple
papers have extended the protocol to prove its security. However, these works
are limited to the case where the two channels have equal amount of loss (i.e.
are symmetric). In a practical network setting, it is very likely that the
channels are asymmetric due to e.g. geographical locations. In this paper we
extend the "simple TF-QKD" protocol to the scenario with asymmetric channels.
We show that by simply adjusting the two signal states of the two users (and
not the decoy states) they can effectively compensate for channel asymmetry and
consistently obtain an order of magnitude higher key rate than previous
symmetric protocol. It also can provide 2-3 times higher key rate than the
strategy of deliberately adding fibre to the shorter channel until channels
have equal loss (and is more convenient as users only need to optimize their
laser intensities and do not need to physically modify the channels). We also
perform simulation for a practical case with three decoy states and finite data
size, and show that our method works well and has a clear advantage over prior
art methods with realistic parameters.
