  The twin-field (TF) quantum key distribution (QKD) protocol and its variants
are highly attractive because they can beat the well-known rate-loss limit
(i.e., the PLOB bound) for QKD protocols without quantum repeaters. In this
paper, we perform a proof-of-principle experimental demonstration of TF-QKD
based on the protocol proposed by Curty et al. which removes from the original
TF-QKD scheme the need for post-selection on the matching of a global phase,
and can deliver nearly an order of magnitude higher secret key rate.
Furthermore, we overcome the major difficulty in the practical implementation
of TF-QKD, namely, the need to stabilize the phase of the quantum state over
kilometers of fiber. A Sagnac loop structure is utilized to ensure excellent
phase stability between the different parties. Using decoy states, we
demonstrate secret-key generation rates that beat the PLOB bound when the
channel loss is above 40 dB.
