Abstract The hope for a futuristic global quantum internet that provides robust and high-capacity quantum information transfer lies largely on qudits, the fundamental quantum information carriers prepared in high-dimensional superposition states. However, preparing and manipulating N-dimensional flying qudits as well as subsequently establishing their entanglement are still challenging tasks, which require precise and simultaneous maneuver of 2 (N-1) parameters across multiple degrees of freedom. Here, using an integrated approach, we explore the synergy from two degrees of freedom of light, spatial mode and polarization, to generate, encode, and manipulate flying structured photons and their formed qudits in a four-dimensional Hilbert space with high quantum fidelity, intrinsically enabling enhanced noise resilience and higher quantum data rates. The four eigen spin–orbit modes of our qudits possess identical spatial–temporal characteristics in terms of intensity distribution and group velocity, thereby preserving long-haul coherence within the entirety of the quantum data transmission link. Judiciously leveraging the bi-photon entanglement, which is well preserved in the integrated manipulation process, we present versatile spin–orbit cluster states in an extensive dimensional Hilbert space. Such cluster states hold the promise for quantum error correction which can further bolster the channel robustness in long-range quantum communication.
Abstract The hope for a futuristic global quantum internet that provides robust and high-capacity quantum information transfer lies largely on qudits, the fundamental quantum information carriers prepared in high-dimensional superposition states. However, preparing and manipulating N-dimensional flying qudits as well as subsequently establishing their entanglement are still challenging tasks, which require precise and simultaneous maneuver of 2 (N-1) parameters across multiple degrees of freedom. Here, using an integrated approach, we explore the synergy from two degrees of freedom of light, spatial mode and polarization, to generate, encode, and manipulate flying structured photons and their formed qudits in a four-dimensional Hilbert space with high quantum fidelity, intrinsically enabling enhanced noise resilience and higher quantum data rates. The four eigen spin–orbit modes of our qudits possess identical spatial–temporal characteristics in terms of intensity distribution and group velocity, thereby preserving long-haul coherence within the entirety of the quantum data transmission link. Judiciously leveraging the bi-photon entanglement, which is well preserved in the integrated manipulation process, we present versatile spin–orbit cluster states in an extensive dimensional Hilbert space. Such cluster states hold the promise for quantum error correction which can further bolster the channel robustness in long-range quantum communication.
Quantum key distribution offers a promising avenue for establishing secure communication networks. However, its performance is significantly hampered by the conventional two-level information carriers (i.e., qubits) due to their limited information capacity and noise resilience. A fundamental approach to overcoming these limitations involves the adoption of high-dimensional qudits. Practical qudit platforms require robust propagation, outstanding controllability, and extreme compactness, to which integrated photonics provides a promising solution. Here, we achieved, for the first time, microlaser-enabled high-dimensional quantum communication through leveraging spin-orbit photon qudits, where the dynamical generation and manipulation of these multi-degrees-of-freedom complex quantum state are realized by a non-Hermitian-physics-driven integrated microlaser quantum transmitter. Such a microlaser photon manipulation, as a novel route towards high-dimensional quantum state generation, promises high energy efficiency, along with fast, compact, and precise qudit state reconfigurability. The four spin-orbit eigenstates emitted by the microlaser possess the same spatial-temporal structures, ensuring homogeneity between all qudit states used for key distribution, which effectively eliminates propagation dephasing and walk-off problems, thereby delivering the high-dimensional spin-orbit secret key generation to construct a robust quantum link. The demonstrated long-term system stability showcases the practical potential of the microlaser quantum transmitter, providing a critical step towards compact, high-information-capacity quantum communication networks.
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.
Twisted photons, carrying spin and orbital angular momentum (OAM) in an unbounded-dimensional Hilbert space, has recently sparked significant interest, leading to the discovery of new physics in both classical and quantum optics, furthering our understanding and incubating applications in classical and quantum communication. At the heart of this emerging field is the requirement for versatile, efficient, and robust generation of novel photonic spin-orbit states involving both spin and OAM degrees of freedom (DoFs). However, so far, the generation and manipulation of spin–orbit photons have primarily relied on free-space bulk optics, which, due to their lack of compactness and stability, are unsuitable for large-scale applications. In this work, we address this challenge by dynamically generating and manipulating the photonic spin-orbit states in both integrated microlaser and single photon source systems, leveraging non-Hermitian photonics and chiral quantum optics. We further investigated the generation and control of quantum entanglement between the spin and OAM DoFs of photons via integrated photonic circuits. Moreover, we successfully generated photonic spin-orbit states with intricate Skyrmionic topological structures and demonstrate the robustness of their topological properties during the quantum teleportation process. The integrated spin-orbit control toolbox for various structured photons, combined with the robustness of their topological properties, holds significant potential for developing large-channel capacity, noise-resilient information systems within a unique multidimensional space.
The ability to prepare, control, and detect high-dimensional photonic quantum states is a key capability in modern quantum science. Spatially structured single- and bi-photon states provide access to large Hilbert spaces, enabling both fundamental studies and practical advances across quantum technologies. Precise control of these states is essential to characterising quantum light sources and to using spatial degrees of freedom for information encoding, with applications ranging from quantum imaging and simulation to high-capacity, noise-resilient quantum communication. In this talk, I will review recent experimental and theoretical progress in structured-light quantum information processing. I will discuss high-dimensional QKD with structured photons, strategies based on adaptive optics and tailored measurements to counter channel distortions, and protocols that exploit spatial mode structure. I will also describe how structured photonic states can be used to simulate linear and cyclic quantum systems, illustrating their potential as scalable platforms for high-dimensional quantum information processing.
High-dimensional entanglement with larger Hilbert spaces enable an encoding of more bits per photon and thus promise increased communication capacities over quantum channels. Quantum frequency combs, which are intrinsically multimode in the temporal and frequency degrees of freedom within a single spatial mode, naturally facilitating the generation and measurement of high-dimensional entanglement. Current challenges include the extension of well-known methods for two qubits to high-dimensional quantum systems and their application in entanglement experiments with photons. More specifically, the major challenge is the certification of high-dimensional entanglement by a number of accessible experimental measurements. In this paper, we increase the Hilbert space dimensionality and provide versatile tools for quantifying and certifying high-dimensional entanglement in a biphoton frequency comb. We quantify the time binned Schmidt number up to 18 and certify entanglement of formation with 1.89 ebits. We have demonstrated a 648- dimensional Hilbert spaces with time-frequency entanglement in a biphoton frequency comb, enabling a computational space up to 13 photonic qubits, and 6.28 bits/photon classical information capacity. This high-dimensional time frequency multimode quantum states of biphoton frequency comb significantly boosting the photon information capacity that is critical for large-scale quantum information processing. Biphoton frequency comb has indeed demonstrated an attractive and powerful approach towards achieving this fundamental goal with applications in high-dimensional quantum information processing, time-frequency cluster-state quantum computation, high-dimensional encoding in quantum networks, and high-dimensional quantum simulations.