  Quantum-dot-based single-photon sources are key assets for quantum
information technology, supplying on-demand scalable quantum resources for
computing and communication. However, longlasting issues such as limited
long-term stability and source brightness have traditionally impeded their
adoption in real-world applications. Here, we realize a quantum key
distribution field trial using true single photons across an 18-km-long dark
fibre, located in the Copenhagen metropolitan area, using an optimized,
state-of-the-art, quantum-dot single-photon source frequency-converted to the
telecom wavelength. A secret key generation rate of >2 kbits/s realized over a
9.6 dB channel loss is achieved with a polarization-encoded BB84 scheme,
showing remarkable stability for more than 24 hours of continuous operation.
Our results highlight the maturity of deterministic single-photon source
technology while paving the way for advanced single-photon-based communication
protocols, including fully device-independent quantum key distribution, towards
the goal of a quantum internet.
