Quantum key distribution (QKD) represents a paradigm shift in secure communication, as it enables the establishment of cryptographic keys with information-theoretic security based on fundamental principles of quantum mechanics. As such, QKD is of great significance for ensuring information security, especially in the current era of increasing data exchange volume and frequency. Despite its promise, the widespread practical deployment of QKD still faces several persistent challenges. These include limited noise resistance, limited communication bandwidth, and difficulties in network compatibility.

To overcome these limitations, high-dimensional quantum key distribution (HD-QKD) has emerged as a powerful solution, offering enhanced performance in several key dimensions. First, the HD states have higher information capacity, so each photon can encode more information, thereby improving the transmission efficiency of information. Second, early research has shown that high-dimensional systems exhibit excellent noise tolerance, whether it is environmental or resulting from eavesdropping attacks. Third, high-dimensional quantum states exhibit richer variations in higher-dimensional Hilbert spaces, which can greatly expand potential application boundaries. Recently, this feature has been used to explore the implementation of multi-mode compatible networks. In this review, we present an overview of the state-of-the-art developments in HD-QKD technologies. We discuss the current state of development and the challenges associated with various technological approaches.

HD-QKD based on temporal, spatial, and hybrid degrees of freedom (DoFs) has advanced significantly, yet faces technical and security challenges. In time-domain encoding, early two-dimensional phase-coding systems utilized Mach-Zehnder interferometers (MZIs) for time-bin states, while energy-time entanglement-based HD-QKD, pioneered by Ali-Khan et al. (2007), demonstrated multi-bit transmission potential. In 2013, Mower et al. integrated security analysis tools for continuous-variable QKD (CV-QKD) and provided an upper bound on information leakage under collective attacks. Subsequent works by Zhang et al. integrated conjugate Franson interferometry, enabling experimentally observable security monitoring. In addition, Islam et al. proposed a comprehensive theoretical framework for HD discrete-variable QKD (DV-QKD) systems, taking into account security aspects including finite-size effects. They also implemented four-dimensional time-bin QKD experiments using cleverly designed cascaded asymmetric MZIs. Unfortunately, due to the difficulty of operating the degree of freedom in the time domain, this encoding method often suffers from low decoding efficiency.

Spatial-mode encoding via orbital angular momentum (OAM) offers high-dimensional Hilbert spaces but struggles with mode stability and fiber compatibility. Breakthroughs in specialty fibers (e.g., ring-core and hollow-core fibers) enabled OAM transmission over 25 km, while free-space experiments validated urban feasibility. However, slow spatial light modulators (SLMs) hinder real-time OAM manipulation. Path encoding via multi-core fibers (MCFs) achieved 4D entanglement distribution over 11 km and hybrid time-path systems over 52 km, highlighting scalability for telecom networks. However, spatial-mode-based encoding still faces significant challenges in extending transmission distances, and phase disturbances in long-distance channels remain difficult.

HD-QKD systems based on single degrees of freedom face practical constraints. Combining multiple degrees, such as polarization and OAM, expands the Hilbert space and eases technical challenges. In 2019, Han’s team utilized vector vortex photon states to map OAM manipulations onto polarization, enabling high-fidelity, low-error HD-QKD (0.60% error rate). The subsequent works extended communication distances to 25 km, enhancing system stability and reducing real-time errors. The scheme is downward-compatible with lower-dimensional QKD, supporting flexible quantum networking and advancing scalable quantum communication infrastructure.

Experimental realization of entangled HD-QKD systems faces significant complexity beyond 2D implementations, currently remaining in early exploration stages. Spontaneous parametric down-conversion (SPDC) serves as the primary method for generating high-dimensional entanglement, utilizing DOFs like OAM, path, or energy-time, or by creating hyperentanglement across multiple DOFs simultaneously. Key challenges include ensuring high-fidelity entanglement distribution and mutually unbiased basis measurements, with each DOF presenting inherent limitations: OAM suffers from spatial mode transmission issues, path requires complex phase stabilization, and time encoding involves difficult measurements.

OAM entanglement states, while theoretically unlimited in dimension (demonstrated up to 10010 dimensions), struggle with distribution (limited to 3 km in free space or 1 km in few-mode fiber) and require complex measurement techniques like phase plates or multi-plane light conversion. Path encoding allows convenient high-dimensional state preparation and measurement via optical elements but relies on multi-core fiber for transmission, demanding active phase compensation over distance. Time encoding performs well in robustness and single-mode fiber compatibility, achieving distribution over hundreds of kilometers. However, measurement typically necessitates complex interferometers, though QKD requirements simplify this somewhat.

Hybrid encoding via hyperentanglement leverages SPDC’s natural ability to entangle multiple DOFs, mitigating individual DOF limitations. Systems based on polarization and time-bin hyperentanglement are fiber-compatible and simplify measurements, enabling 50 km HD-QKD demonstrations. Beyond QKD, hyperentanglement enables advanced protocols like efficient single-copy entanglement purification. This technique uses hyperentangled states to perform deterministic CNOT operations within a single photon pair, thereby purifying one DOF against depolarization noise. It vastly outperforms traditional two-copy purification in efficiency. Experimental implementations by Hu et al. and Ecker et al. in 2021 successfully demonstrated fidelity boosts and restored QKD performance over noisy links, proving its vital role in enabling practical long-distance quantum communication.

In summary, HD-QKD using the weak coherent sources shows increasing dimensions and rates, surpassing 2D-QKD performance in some aspects. However, practical adoption faces significant hurdles. Time-phase encoding offers compatibility but suffers from efficiency loss and extended time slots at higher dimensions. Spatial mode encodings avoid time issues but lack standard fiber compatibility and face atmospheric distortion in free space, requiring advanced wavefront correction. Multicore fiber is promising for path encoding but needs improved phase stabilization. Fortunately, with advances in experimental techniques, the limitations in each DOF are gradually being mitigated, though considerable progress is still required. HD entanglement offers enhanced capabilities and noise resilience for quantum communications. In particular, recent subspace encoding protocols have significantly improved the noise resistance and environmental adaptability of HD entangled QKD, offering a promising technological pathway for deploying QKD in complex environments. In addition, HD systems based on hyperentanglement also offer advantages in protocols such as entanglement purification and entanglement swapping, which can improve the operational efficiency of these processes. Experimentally, research on HD entangled QKD is gradually transitioning from short-range proof-of-principle demonstrations to systems capable of supporting long-distance transmission. However, high-dimensional quantum states are highly susceptible to channel noise, and achieving high-fidelity transmission and measurement over long distances remains one of the key technical challenges in this field. Fortunately, with the continuous development of theory and experiments, we believe that in the near future, HD-QKD will soon demonstrate its advantages in practical applications and play a key supporting role in the future development and upgrading of quantum networks.