  Twin-field quantum key distribution (TF-QKD) overcomes the linear rate-loss
limit, which promises a boost of secure key rate over long distance. However,
the complexity of eliminating the frequency differences between the independent
laser sources hinders its practical application. Here, taking the saturated
absorption spectroscopy of acetylene as an absolute reference, we propose and
demonstrate a simple and practical approach to realize TF-QKD without requiring
relative frequency control of the independent laser sources. Adopting the
4-intensity sending-or-not-sending TF-QKD protocol, we experimentally
demonstrate the TF-QKD over 502 km, 301 km and 201 km ultra-low loss optical
fiber respectively. We expect this high-performance scheme will find widespread
usage in future intercity and free-space quantum communication networks.
