Fig. 1. Four-wave mixing in optical fibers. (a) Schematic of spontaneous four-wave mixing process in optical fiber; (b) calculation results of phase matching wavelength of four-wave mixing in photonic crystal fiber with zero dispersion wavelength λ₀ of 1062 nm; (c) Raman scattering gain spectrum in fused quartz (solid line and dotted line represent Raman scattering gain parallel and perpendicular to pump polarization, respectively)^[22]

Fig. 2. Schematic of joint spectral function of photon pairs in different situations. (a) Positive frequency correlation; (b) inverse frequency correlation; (c) no frequency correlation

Fig. 3. Experimental apparatus and measurements for preparing correlated photon pairs. (a) Schematic of experimental apparatus for preparing correlated photon pairs using straight optical fiber; (b) schematic of experimental apparatus for preparing correlated photon pairs using Sagnac fiber ring structure; (c) variation trend of experimental coincidence counting rate and random coincidence counting rate with single channel photon counting rate^[28]; (d) variation curves of CAR with pump peak power at different fiber temperatures^[29]

Fig. 4. Relationship between correlated photon pairs and pump power. (a) Coincidence count rate of correlated photon pairs from photonic crystal fiber as function of average pump power, illustrated shows cross section of photonic crystal fiber^[36]; (b) coincidence count rate and CAR of correlated photon pairs from micro/nano-fiber as functions of average pump power, illustration shows micro/nano fiber structure^[38]

Fig. 5. Spectrum of correlated photon pairs in photonic crystal fibers^[44]. (a) Theoretical calculation results of spectrum of correlated photon pairs in photonic crystal fibers; (b) HOM interference fringe patterns between independent light sources measured experimentally

Fig. 6. Schematic of nonlinear fiber optic interferometer and its experimental measurement results. (a) Schematic of N-class (N≥2) nonlinear optical fiber interferometer^[46]; (b) associated photon pair spectrum of output of three-stage nonlinear fiber interferometer measured experimentally^[47]; (c) HOM interference fringe pattern between independent light sources (left and right are original results and results after modified Raman photon and multi-photon events respectively)^[47]

Fig. 7. Generation principle and experimental results of polarization entangled photon pairs. (a) Schematic of polarization entangled photon pairs produced by two vertically polarized pump light co-pumped fiber^[50]; (b) variation curves of single-channel counting rate and coincidence counting rate in Fig. (a) with angle between polarizers^[50]; (c) schematic of polarization entangled photons produced by optical fiber ring directional pumping structure^[51]; (d) variation curves of coincidence counting rate of Fig. (c) with angle between polarizers^[51]

Fig. 8. Generation device and experimental measurement results of Time-bin entangled photon^[59]. (a) Schematic of Time-bin entangled photon generation device; (b) experimentally measured curves of coincidence counting rate with temperature of interferometer

Fig. 9. Structure and experimental measurement results of Sagnac fiber ring^[62]. (a) Frequency entanglement generation and measurement device based on Sagnac fiber ring structure; (b) frequency entanglement interference beat fringes measured by experiment