Fig. 1. Time-domain coding HD-QKD experiments. (a) HD-QKD experiment based on energy-time entanglement^[40]; (b) HD-QKD based on dispersive optics^[41]; (c) HD-QKD based on the Franson interferometer and the conjugate Franson interferometer^[42]

Fig. 2. OAM-encoded HD-QKD experiments. (a) HD-QKD experiment over a 300 m free-space channel in an urban environment^[48]; (b) HD-QKD experiment in a 1.2 km hollow-core fiber^[49]; (c) HD-QKD based on polarization-OAM mutual mapping in a 25 km high-refractive-index ring-core fiber channel^[50]

Fig. 3. Path-encoded HD-QKD experiments. (a) 4-dimensional path-encoded HD-QKD experiment over a 300 m multi-core fiber^[54]; (b) path-encoded silicon photonic integrated HD-QKD based on a 3 m long 4-core fiber channel^[55]; (c) high-dimensional entanglement distribution and HD-QKD experiment with hybrid polarization-path encoding in an 11 km multi-core fiber channel^[56]; (d) time-path hybrid encoding HD-QKD experiment in a 52 km (26 km fiber loop) field-deployed fiber^[60]

Fig. 4. HD-QKD experimental systems based on multi-degree-of-freedom joint encoding. (a) Schematic diagram of the 4D-QKD photon state with polarization-OAM joint encoding^[64]; (b) HD-QKD based on a 25 km ring-core fiber^[50]; (c)(d) 8D-QKD and multi-degree-of-freedom compatible HD-QKD network based on the joint encoding of polarization, time-phase, and OAM^[65]

Fig. 5. Preparation method for high-dimensional OAM entanglement

Fig. 6. Preparation method for path entanglement

Fig. 7. Preparation methods for time-encoded high-dimensional entanglement. (a) Schematic diagram of energy-time entanglement preparation; (b) schematic diagram of high-dimensional time-bin entanglement preparation

Fig. 8. Preparation method for polarization-time-bin hyperentanglement

Fig. 9. Schematic diagrams of entanglement purification steps^[96]. (a) Two-copy protocol; (b) single-copy protocol

Fig. 10. Experimental setup for single-copy entanglement purificationin the work of Hu et al^[59]. (a) Generation of hyperentangled photon pairs; (b) quantum channel subject to depolarizing noise; (c) entanglement purification stage; (d) apparatus for path quantum state projection measurement

Fig. 11. Experimental setup for single-copy entanglement purification in the work of Ecker et al^[96]