Fig. 1. Two implementations of QKD protocol. (a) Schematic diagram of preparation-measurement-based scheme; (b) schematic diagram of entanglement-based scheme

Fig. 2. Multiple potential degrees of freedom for constructing high-dimensional quantum states^[33]

Fig. 3. Schematic diagrams of OAM intensity and phase distribution

Fig. 4. HD-QKD experiments based on OAM. (a) Proof-of-principle demonstration of 7-dimensional QKD based on DMD^[49]; (b) 4-dimensional HD-QKD experiment in free-space link of 300 m over the city^[16];^{(c) HD-QKD experiment based on polarization-OAM mutual mapping[31,52]; (d) MPUB-based HD-QKD protocol experiment[56-57]}

Fig. 5. Schematic diagram of preparation for high-dimensional Time-bin entangled state^[22]

Fig. 6. Experiments based on Time-bin HD-QKD. (a) Preparation-measurement-based 4-dimensional Time-bin QKD scheme^[20]; (b) quantum measurement scheme based on two-photon interference^[65];^{(c) large-alphabet Time-bin QKD experimental device[68]; (d) compact 4-dimensional measurement scheme[66]; (e) Time-bin-path hybrid coding experiment using multi-core fiber[70]; (f) high-dimensional Time-bin integrated photonic platform[71]}

Fig. 7. Preparation-measurement HD-QKD experiments based on path coding. (a) Decoy-state HD-QKD experiment based on multicore fiber^[17]; (b) path coding HD-QKD experiment implemented on integrated circuit ^[78]; (c) high-dimensional MDI-QKD scheme^[79]; (d) HD-QKD experiment based on PLL^[80]

Fig. 8. High-dimensional entanglement distribution experiments based on path coding. (a) 4-dimensional path-polarization super entanglement distribution experiment on 11 km multicore fiber^[81]; (b) 8-dimensional entanglement distribution experiment based on subspace coding^[82]; (c) 4-dimensional entanglement distribution experiment based on multi-chip quantum network^[83]

Fig. 9. Experimental progresses of high-dimensional multi-degree of freedom quantum networks. (a) Conceptual diagram of composable quantum networks^[86]; (b) three-degree of freedom (Time-bin, polarization, and spatial mode) hybrid 8-dimensional QKD system^[86]; (c) experimental setup of simultaneous transmission for three-degree of freedom super entanglement in multicore fiber^[87]; (d) 2×N plug and play TF-QKD network scheme^[88]

Table 1. Comparative analysis for degrees of freedom in HD-QKD: advantages, challenges, and application scenarios

Table 2. HD-QKD experimental indicators