Doughnut-biting photon capturing scheme for drone-based orbital angular momentum quantum key distribution.

Drone-based quantum key distribution (QKD) offers a flexible, cost-effective, and reconfigurable approach to extending the reach of spatial and temporal quantum communication. The rotation-invariant properties of orbital angular momentum (OAM) effectively mitigate issues related to reference frame alignment from the drone platform. Utilizing OAM encoding to achieve high-dimensional quantum key distribution (HD-QKD) exhibits significant advantages in terms of communication capacity and robustness. However, the state-dependent diffraction of OAM requires a single large-aperture receiving telescope, which restricts the communication distance of drone-based QKD. In this paper, we propose a "doughnut-biting" photon-capturing scheme for drone-based OAM-QKD. Firstly, a mobile model for air-to-air OAM-encoded QKD is established based on a drone platform. Secondly, the performance of the system under center-aligned (CA) and center-misaligned (CM) receiving schemes based on the intensity distribution of OAM are compared. Numerical simulations indicate that the CA scheme offers simpler targeting technology and higher information capacity at short distances, while the CM scheme extends the transmission distance and provides advantages in data rate. In conclusion, an efficient "doughnut-biting" scheme is proposed to increase the transmission distance by at least 50.5% in the receiving aperture range of 5-20 cm. The proposed scheme provides a practical framework for implementing long-distance OAM-encoded QKD in free space, contributing to the development of an integrated space-to-ground quantum communication network. We introduce space-time metamaterials as the natural evolution of time-varying metamaterials, highlighting their enhanced properties and potential advantages. These metamaterials offer virtually limitless diversity, driven by their dynamic levels, velocity regimes, and space-time architectures. Notably, it unlocks extensive possibilities for transition engineering-the precise control of classical and quantum state transitions through tuning modulation velocity, potential, or dispersion.