  Quantum key distribution (QKD) enables two remote parties to share encryption
keys with security based on the laws of physics. Continuous variable (CV) QKD
with coherent states and coherent detection integrates well with existing
telecommunication networks. However, thus far, long-distance CV-QKD has only
been demonstrated using a highly complex scheme where the local oscillator is
transmitted, opening security loopholes for eavesdroppers and limiting its
potential applications. Here, we report a long-distance CV-QKD experiment with
a locally generated local oscillator over a 100 km fiber channel with a total
loss of 15.4 dB. This record-breaking distance is achieved by controlling the
phase-noise-induced excess noise through a machine-learning framework for
carrier recovery and optimizing the modulation variance. We implement the full
CV-QKD protocol and demonstrate the generation of keys secure against
collective attacks in the finite-size regime. Our results mark a significant
milestone for realizing CV quantum access networks with a high loss budget, and
pave the way for large-scale deployment of secure QKD.
