Fig. 1. Schematics of the experimental setup in the Nanjing University optical fiber network. Node A and node B are located in the Zhongying Tang Building and the Electron Microscope Building, respectively, in the Gulou Campus. Node C is located in the Fundamental Laboratory Building in the Xianlin Campus. These nodes are separated by distances of 0.2 km and 30.5 km. Fiber is installed along the yellow line. Abbreviations of components: IM, intensity modulator; IM bias, intensity modulator bias; AMP, amplifier; PM, phase modulator; PBS, polarization beam splitter; BS, beam splitter; PC, polarization controller; EPC, electrical polarization controller; DWDM, dense wavelength division multiplexer; SynLs, synchronized laser; FM, Faraday rotation mirror; PS, phase shifter; SSPD, superconducting single-photon detector; PD, power detector; APD, avalanche photodiode FPGA; field programmable gate array; VDC, variable direct current. Imagery©2020 Google. Map data from Google, Maxar Techonologies, CNES/Airbus.

Fig. 2. Characterization of the quantum channel. (a) Density matrices of output time-bin-encoded states. (b) State fidelities of the six output states to the ideal states. (c), (d) Real and imaginary parts of the process matrices for the quantum channel with a fidelity of F0=99.3%±0.7%. (e) Bloch sphere representation of the process. The plot shows how the ideal states on the surface of the Bloch sphere (meshed) are influenced by the quantum channel, with the output states lying on the solid surface. The uncertainties in state fidelities are calculated using a Monte Carlo routine assuming Poissonian error.

Fig. 3. A 12 h continuous operation of the Nanjing quantum network with excellent system parameters: quantum bit error rate (blue, left vertical axis) and (red, right vertical axis) for COW protocol.

Fig. 4. Field trial SKR as a function of attenuation. Green and red pentagrams are our SKR on the network.