  Single-photon sources are essential for quantum networks, enabling
applications ranging from quantum key distribution (QKD) to the burgeoning
quantum internet. Despite the remarkable advancements, the current reliance of
QKD on attenuated coherent (laser) light sources has imposed a fundamental
limit on the secret key rate (SKR). This constraint is primarily attributable
to the scarcity of single-photon components within coherent light, confined by
an inherent upper bound of 1/e. Here, we report high-rate QKD using a
high-efficiency single-photon source, enabling an SKR transcending the
fundamental rate limit of coherent light. We developed an on-demand, bright
semiconductor quantum-dot single-photon source with an efficiency of 0.71(2),
exceeding the inherent bound of coherent light by approximately 2.87 dB.
Implementing narrow-bandwidth filtering and random polarization modulation, we
conducted a field QKD trial over a 14.6(1.1)-dB-loss free-space urban channel,
achieving an SKR of 0.00108 bits per pulse. This surpasses the practical limit
of coherent-light-based QKD by 2.53 dB. Our findings conclusively demonstrate
the superior performance of nanotechnology-based single-photon sources over
coherent light for QKD applications, marking a pivotal stride towards the
realization of a global quantum internet.
