Abstract A high‐dimensional quantum network with multiple degrees of freedom provides enhanced channel capacity and noise resilience, making it highly significant for quantum communication. However, achieving quantum networks with high‐dimensional hyper‐entanglements using traditional optical systems presents substantial challenges. In this study, the generation of a compact high‐dimensional hyper‐entangled photon source utilizing a metasurface is demonstrated. Furthermore, a scalable protocol for the remote preparation of high‐dimensional single‐photon states, ranging from 3 to 8 dimensions based on this entangled light source is theoretically proposed and experimentally validated. The average fidelities of the remotely prepared states all exceed 0.9. This metasurface‐based approach to quantum communication protocols presents a promising avenue for the development of large‐scale quantum networks and distributed quantum computing systems.
Abstract A high‐dimensional quantum network with multiple degrees of freedom provides enhanced channel capacity and noise resilience, making it highly significant for quantum communication. However, achieving quantum networks with high‐dimensional hyper‐entanglements using traditional optical systems presents substantial challenges. In this study, the generation of a compact high‐dimensional hyper‐entangled photon source utilizing a metasurface is demonstrated. Furthermore, a scalable protocol for the remote preparation of high‐dimensional single‐photon states, ranging from 3 to 8 dimensions based on this entangled light source is theoretically proposed and experimentally validated. The average fidelities of the remotely prepared states all exceed 0.9. This metasurface‐based approach to quantum communication protocols presents a promising avenue for the development of large‐scale quantum networks and distributed quantum computing systems.
Abstract High‐dimensional quantum states, or ‘qudits’, provide significant advantages over traditional qubits in quantum communication, such as increased information capacity, enhanced noise resilience, and reduced information loss. Despite these benefits, their implementation has been constrained by challenges in generation, transmission, and detection. This paper presents a novel theoretical framework for transmitting quantum information using qudit entanglement distribution over a superposition of causal orders in two quantum channels. Using this model, a quantum switch operation for 2‐qudit systems is introduced, which facilitates enhanced fidelity of entanglement distribution and quantum teleportation. The results demonstrate that the use of qudits in entanglement distribution achieves a fidelity improvement from 0.5 (for qubit‐based systems) to 0.94 for 20‐dimensional qudits, even under noisy channel conditions. This enhancement is achieved by exploiting the increased Hilbert space of high‐dimensional states and the inherent noise‐resilience properties of quantum switches operating in superpositions of causal orders. The findings underscore the potential of qudit‐based quantum systems in achieving robust and high‐fidelity communication in environments where traditional qubit‐based systems face limitations.
The field of high-dimensional quantum photonics involves the use of multimode photonic degrees-of-freedom such as the spatial, temporal, or spectral structure of light to encode multi-level quantum states. Recent years have seen rapid progress in the development of methods to generate, manipulate, and distribute such quantum states of light and their use in a range of quantum technology applications that offer practical advantages over conventional qubit-based approaches. High-dimensional quantum states of light encoded in photonic time-bins, frequency-bins, transverse-spatial modes, waveguide paths, and temporal modes have enabled noise-robust fundamental tests of quantum mechanics, error-resilient and high-capacity quantum communication protocols, andas well as efficient approaches for quantum information processing, to name just a few examples. However, research in this field has progressed fairly independently, with little exchange across different photonic degrees-of-freedom or between experiment and theory and no comprehensive comparison between degrees-of-freedom. This roadmap aims to bridge this gap by surveying progress in each area and identifying shared challenges and opportunities that cut across two or more photonic degrees-of-freedoms. We review early work and state-of-the-art experimental techniques under development for high-dimensional quantum states encoded in single and entangled photons, as well as theoretical tools for their measurement and certification. We outline the main outstanding challenges for theory and each experimental degree-of-freedom, identifying promising future directions of research that may enable these to be overcome. We end by discussing interconnections and shared challenges centered around their distribution, measurement, and manipulation, with a view towards their integration into next-generation quantum technology platforms and applications.
Conventional approaches to generating and distributing entangled quantum states usually rely on bulky traditional optical devices, posing significant challenges to system miniaturization and integration. Here we report an entanglement engineering scheme that utilizes quantum interference on a metasurface, combined with postselection, to generate and distribute multiple polarization-entangled photon pairs. Two unentangled identical photons, differing only in polarization, are incident on a multichannel metasurface where each channel supports a specific polarization transformation. The metasurface facilitates two-photon interference, generating postselected polarization-entangled states and forming a fully connected entanglement distribution among every two output channels. Experiments demonstrate the generation and distribution of four Bell states across 21 channel pairs, underscoring the potential of metasurfaces as compact, scalable entanglement sources for quantum networking.
Abstract In recent years, there has been a rising interest in high‐dimensional quantum states and their impact on quantum communication. Indeed, the availability of an enlarged Hilbert space offers multiple advantages, from larger information capacity and increased noise resilience, to novel fundamental research possibilities in quantum physics. Multiple photonic degrees of freedom have been explored to generate high‐dimensional quantum states, both with bulk optics and integrated photonics. Furthermore, these quantum states have been propagated through various channels, for example, free‐space links, single‐mode, multicore, and multimode fibers, and also aquatic channels, experimentally demonstrating the theoretical advantages over 2D systems. Here, the state‐of‐the‐art on the generation, propagation, and detection of high‐dimensional quantum states is reviewed. Quantum communication with states living in d ‐dimensional Hilbert spaces, qudits, yields great benefits. However, qudits generation, transmission, and detection is not a simple task to accomplish. This review presents the state‐of‐the‐art on the generation, propagation, and measurement of high‐dimensional quantum states, highlighting their advantages, issues, and future perspectives.
High-dimensional photonic entanglement holds significant promise for advancing quantum communication, computation, and metrology. For example, large-alphabet quantum communication protocols are known to benefit from enhanced noise resilience and information capacity via multi-bit time-bin encoding. Yet, characterizing high-dimensional entangled states is challenging, as full state tomography becomes prohibitively costly and often requires unrealizable measurements. Here, we demonstrate a scan-free method to characterize high-dimensional entanglement in the time-frequency domain. Our reconstruction achieves a record 5.70±0.07 ebits and a fidelity of 65.4±0.4% with the maximally entangled state of local dimension 1021, certifying the presence of 668-dimensional entanglement. We further prove the attainability of a secure key rate of 15.6 kB/s in a composable finite-size, entanglement-based protocol, and show that in continuous operation, the setup can quickly approach asymptotic key rates. Using commercial telecom components and state-of-the-art low-jitter single-photon detectors, our scalable architecture offers a practical path towards high-rate, noise-resilient quantum communication testbeds.
Abstract High-dimensional quantum entanglement is an important resource for emerging quantum technologies such as quantum communication and quantum computation. The scalability of metres-long experimental setups limits high-dimensional entanglement in bulk optics. Advancements in quantum technology hinge on reproducible, and reconfigurable quantum devices—including photon sources, which are challenging to achieve in a scalable manner using bulk optics. Advances in nanotechnology and CMOS-compatible integration techniques have enabled the generation of entangled photons on millimeter-scale chips, significantly enhancing scalability, stability, replicability, and miniaturization for real-world quantum applications. In recent years we have seen several chip-scale demonstrations with different discrete degrees of freedom including path, frequency-bin, time-bin, and transverse modes, on many material platforms. A complete quantum photonic integrated circuit requires the generation, manipulation, and detection of quantum states, involving various active and passive quantum photonic components which further increase the degree of complexity. Here, we focus on the high-dimensional versions of qubits—qudits—and review the nonlinear optical processes that facilitate on-chip high-dimensional entangled photon sources, and the currently used material platforms. We discuss a range of current implementations of on-chip high-dimensional entangled photon sources and demonstrated applications. We comment on the current challenges due to the limitations of individual material platforms and present future opportunities in hybrid and heterogeneous integration strategies for the next generation of integrated quantum photonic chips.