Decoy-State Quantum Key Distribution over 227 km with a Frequency-Converted Telecom Single-Photon Source.

We implement a decoy-state quantum key distribution scheme using a telecom C-band single-emitter source. The decoy states are created by varying the optical excitation of the quantum emitter to modulate the photon-number distribution. We provide an analysis of our scheme based on existing security proofs, allowing the calculation of secret key rates, including finite key effects. This enables us to demonstrate, with a realistic single-photon source, positive secret key rates using our scheme over 227 km of optical fiber, equivalent to a loss tolerance one order of magnitude greater than nondecoy schemes. This Letter broadens the scope of single-photon sources in future quantum networks by enabling long-distance quantum key distribution with realistic levels of single-photon purity.