Fig. 1. Encoding of the states sent by Alice. Z and X are the bases in which the states |0⟩, |1⟩ and, |+⟩, respectively, live. μ1 and μ2 correspond to the two mean photon numbers used for the one-decoy state protocol [26].

Fig. 2. Simplified schematic of the experimental setup. PIC, photonic integrated circuit; DCF, dispersion compensating fiber; QC, quantum channel; SMF, single-mode fiber; PG, pulse generator; APCB, Alice printed circuit board (PCB); BPCB, Bob PCB; FPGA, field-programmable gate array; SC, service channel. Black lines correspond to optical links, and blue lines correspond to electrical connections.

Fig. 3. Photo of the transmitter integrated circuit.

Fig. 4. Structure of the integrated transmitter circuit. Imb-MZI, imbalanced Mach–Zehnder interferometer; IM, intensity modulator; VOAs, variable optical attenuators; HT, heater; AA, absorption attenuator; EOPS, electro-optic phase shifter; PD, photodiode. The lengths of the three EOPSs are 200 μm, 400 μm, and 600 μm.

Fig. 5. Structure of the receiver integrated circuit. X means non-fiber-coupled ports. Fibers are butt-coupled to the waveguides and permanently pigtailed with UV-curing, index-matching glue. Fiber to waveguide coupling losses are better than 0.3 dB/facet.

Fig. 6. QBERz, ϕz, RKR, and SKR during several secret key exchanges over 80 min using SNSPDs at a distance of 202.0 km SMF.