Preparing and manipulating N-dimensional flying qudits as well as subsequently establishing their entanglement are still challenging tasks. 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 photon qudits in a four-dimensional Hilbert space with high quantum fidelity, intrinsically enabling enhanced noise resilience and higher quantum data rates.
Preparing and manipulating N-dimensional flying qudits as well as subsequently establishing their entanglement are still challenging tasks. 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 photon qudits in a four-dimensional Hilbert space with high quantum fidelity, intrinsically enabling enhanced noise resilience and higher quantum data rates.
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.
Hybrid integration of solid-state quantum emitters (QEs) into nanophotonic structures opens enticing perspectives for exploiting multiple degrees of freedom of single-photon sources for on-chip quantum photonic applications. However, the state-of-the-art single-photon sources are mostly limited to two-level states or scalar vortex beams. Direct generation of high-dimensional structured single photons remains challenging, being still in its infancy. Here, we propose a general strategy to design highly entangled high-dimensional spin-orbital single-photon sources by taking full advantage of the spatial freedom to design QE-coupled composite (i.e., Moiré/multipart) metasurfaces. We demonstrate the generation of arbitrary vectorial spin-orbital photon emission in high-dimensional Hilbert spaces, mapping the generated states on hybrid-order Bloch spheres. We further realize single-photon sources of high-dimensional spin-orbital quantum emission and experimentally verify the entanglement of high-dimensional superposition states with high fidelity. We believe that the results obtained facilitate further progress in integrated solutions for the deployment of next-generation high-capacity quantum information technologies.
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.
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.
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.
Light carries both spin (polarization) and orbital angular momentum. Combining these degrees of freedom produces hybrid spin-orbit states that live in a high-dimensional Hilbert space, offering greater information capacity and robustness for optical communication, quantum technologies, and metrology. However, generating arbitrary states in these spaces and characterizing them efficiently has remained difficult. Here we show a compact metasurface that generates arbitrary spin-orbit states in a four-dimensional Hilbert space, visualized on a Poincaré hypersphere, with straightforward scalability to higher dimensions. Using a tetratomic unit cell, the single-layer device precisely controls complex amplitude, phase, and polarization. We further introduce an efficient interferometric scheme that reconstructs the full density matrix of any N-dimensional spin-orbit state using only three interferograms. This approach uncovers an intrinsic spin-orbit parity order that governs the symmetry of projected intensity patterns, independent of the weighting of the eigenmodes, and enables controlled mode transformations through higher-order geometric phases. These advances establish a versatile platform for high-dimensional photonic technologies.