  Quantum key distribution is a cornerstone of quantum technology, offering
information-theoretical secure keys for remote parties. With many quantum
communication networks established globally, the mode-pairing protocol stands
out for its efficacy over inter-city distances using simple setups, emerging as
a promising solution. In this study, we employ the mode-pairing scheme into
existing inter-city fiber links, conducting field tests across distances
ranging from tens to about a hundred kilometers. Our system achieves a key rate
of $1.217$ kbit/s in a $195.85$ km symmetric link and $3.089$ kbit/s in a
$127.92$ km asymmetric link without global phase locking. The results
demonstrate that the mode-pairing protocol can achieve key rates comparable to
those of a single quantum link between two trusted nodes on the
Beijing-Shanghai backbone line, effectively reducing the need for half of the
trusted nodes. These field tests confirm the mode-pairing scheme's
adaptability, efficiency, and practicality, positioning it as a highly suitable
protocol for quantum networks.
