  Twin-field (TF) quantum key distribution (QKD) is highly attractive because
it can beat the fundamental limit of secret key rate for point-to-point QKD
without quantum repeaters. Many theoretical and experimental studies have shown
the superiority of TFQKD in long-distance communication. All previous
experimental implementations of TFQKD have been done over optical channels with
symmetric losses. But in reality, especially in a network setting, the
distances between users and the middle node could be very different. In this
paper, we perform a first proof-of-principle experimental demonstration of
TFQKD over optical channels with asymmetric losses. We compare two compensation
strategies, that are (1) applying asymmetric signal intensities and (2) adding
extra losses, and verify that strategy (1) provides much better key rate.
Moreover, the higher the loss, the more key rate enhancement it can achieve. By
applying asymmetric signal intensities, TFQKD with asymmetric channel losses
not only surpasses the fundamental limit of key rate of point-to-point QKD for
50 dB overall loss, but also has key rate as high as $2.918\times10^{-6}$ for
56 dB overall loss. Whereas no keys are obtained with strategy (2) for 56 dB
loss. The increased key rate and enlarged distance coverage of TFQKD with
asymmetric channel losses guarantee its superiority in long-distance quantum
networks.
