  Quantum key distribution (QKD) aims to generate secure private keys shared by
two remote parties. With its security being protected by principles of quantum
mechanics, some technology challenges remain towards practical application of
QKD. The major one is the distance limit, which is caused by the fact that a
quantum signal cannot be amplified while the channel loss is exponential with
the distance for photon transmission in optical fiber. Here using the
3-intensity sending-or-not-sending protocol with the
actively-odd-parity-pairing method, we demonstrate a fiber-based twin-field QKD
over 1002 km. In our experiment, we developed a dual-band phase estimation and
ultra-low noise superconducting nanowire single-photon detectors to suppress
the system noise to around 0.02 Hz. The secure key rate is $9.53\times10^{-12}$
per pulse through 1002 km fiber in the asymptotic regime, and
$8.75\times10^{-12}$ per pulse at 952 km considering the finite size effect.
Our work constitutes a critical step towards the future large-scale quantum
network.
