Curator's Take
This comprehensive roadmap addresses a critical gap in quantum photonics by bringing together researchers working on different approaches to encoding quantum information in light's various properties - from spatial patterns to timing and frequency. While most quantum computing discussions focus on qubits, high-dimensional quantum states using photons can carry much more information per particle and offer inherent advantages for certain applications like quantum communication and sensing. The article's cross-pollination approach is particularly valuable because researchers working on spatial modes, temporal encoding, and spectral methods have largely operated in silos despite facing similar challenges. This unified perspective could accelerate progress toward practical photonic quantum technologies that leverage light's rich multidimensional nature rather than forcing it into simple two-level systems.
— Mark Eatherly
Summary
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.