Fig. 1. Quantum key distribution systems based on integrated light source. (a) Experimental setup for single-photon source based on quantum dot^[59]; (b) experimental setup for on-chip entanglement source based on four-wave mixing^[64]; (c) schematic of a multichannel quantum key distribution system based on an optical frequency comb^[65]

Fig. 2. Codecs of quantum key distribution system. (a) Codec for phase degrees of freedom^[71]; (b) encoder for polarization degrees of freedom^[72]; (c) decoder for polarization degrees of freedom^[6]

Fig. 3. Quantum key distribution systems based on integrated superconducting nanowire single-photon detectors. (a) Scanning electron microscope (SEM) image of receiver chip^[10]; (b) microscope image of the receiver chip for four-channel systems^[78]; (c) schematic of the experiment setup for MDI protocols and the SEM image of the chip device^[79]

Fig. 4. Integrated photonic chips compatible with multiple QKD protocols^[54]. (a) Integrated InP transmitter; (b) integrated SiO_xN_y receiver; (c) InP waveguide cross-section; (d) wavelength tunable continuous wave laser; (e) microscopic image of the MZI; (f) SiO_xN_y waveguide cross-section; (g) microscopic image of the receiver delay lines

Fig. 5. Silicon-based optoelectronics QKD chip^[50]. (a) COW protocol; (b) polarization encoded BB84 protocol; (c) time-bin phase encoded BB84 protocol; (d) Bloch ball diagram

Fig. 6. Schematic diagram of the integrated MDI-QKD schemes. (a) Transmitter chip based on InP material^[88]; (b) high-speed quantum state preparation chip based on SOI material^[30]; (c) preparation and measurement chip based on SOI material^[90]

Fig. 7. QKD schemes with miniaturization and high integration. (a) Modulator-free transmitter chip^[87]; (b) QKD modules and chips^[95]; (c) hybrid integrated QKD transceiver and system^[101]

Fig. 8. Integrated QKD schemes resistant to polarization disturbance. (a) Decoder chip with active feedback for polarization variations^[75]; (b) decoder chip of automatically compensating for polarization variations^[97]

Fig. 9. Integrated QKD field trial experimental systems. (a) Aerial view of the 43 km dark fiber link experimental system^[86]; (b) aerial view and setups of the 145 m free space experimental system^[92]

Fig. 10. Laser damage attack on an integrated QKD chip^[120]. (a) Schematic diagram of the InP QKD transmitter chip; (b) damaged spot size converter

Fig. 11. Reflectivity experiments on a silicon-based QKD chip^[93]. (a) Schematic of the silicon-based transmitter chip and reflectivity measurement experiment; (b) reflectivity inside the silicon chip; (c) comparison of reflectivity between the fiber optic modulator and silicon-based chip

Fig. 12. Transmittance-invariant phase modulator^[125]. (a) Schematic diagram of the modulator; (b) microscope image of the modulator

Fig. 13. Schematic diagram of the photorefractive effect procedure^[127]