  Twin-field quantum key distribution (TFQKD), using single-photon-type
interference, offers a way to exceed the rate-distance limit without quantum
repeaters. However, it still suffers from the photon losses and dark counts,
which impose an ultimate limit on its transmission distance. In this letter, we
propose a scheme to implement TFQKD with an entangled coherent state source in
the middle to increase its range, as well as comparing its performance under
coherent attacks with that of TFQKD variants. Simulations show that our
protocol has a theoretical distance advantage of 400 kilometers. Moreover, the
scheme has great robustness against the misalignment error and finite-size
effects. Our work is a promising step toward long-distance secure communication
and is greatly compatible with future global quantum network.
