Fig. 1. Development history of entangled-photon source

Fig. 2. Schematic of process and energy levels of converting high energy photon into two low energy photons^[60]. (a) Process of generating photon pairs after SPDC in pumped second-order nonlinear crystal; (b) schematic of equivalent energy level transition

Fig. 3. Type-II SPDC on BBO crystal, generation of two parametric photons with orthogonal polarization

Fig. 4. Fundamentals of QPM. (a) Principle of quasi-phase-matching using periodic polarization; (b) polarization inversion and quasi-phase-matching in real-world processes

Fig. 5. Satellite-based entangled-photon source of Micius. (a) Main optical path of entangled-photon source^[15]; (b) polarization detection of entangled-photon source^[15]; (c) 3D model diagram of entangled-photon source engineering prototype

Fig. 6. SpooQy-1 experimental scheme^[68]. (a) Important optical components in optical path; (b) schematic of entangled-photon source generated by two BBO crystals; (c) detection devices for entangled-photon pairs generated by SPDC; (d) SpooQy-1 micro-nano satellite structure diagram

Fig. 7. Experimental diagram of teleportation scheme^[73]

Fig. 8. Entangled-photon source engineering prototype of ESA project^[74]

Fig. 9. Setup of type-0 PPKTP entangled-photon source^[21]

Fig. 10. Bell inequality test experiment with human freewill^[21]. (a) Concept diagram of experimental setup; (b) simulation experiment diagram