Symmetrically simplified high-dimensional quantum key distribution

High-dimensional quantum key distribution (HD-QKD) offers significant advantages in photon information capacity and noise tolerance compared to the two-dimensional system. However, the practical deployment of HD-QKD systems is hindered by the high complexity of implementing multiple states for both preparation and measurement. Existing simplification schemes have primarily focused on reducing the number of prepared states at the transmitter, leaving the receiver's measurement complexity unaddressed. In this work, we propose a symmetrically simplified HD-QKD protocol that simultaneously reduces the number of quantum states involved in both the state preparation and measurement processes. We also present a rigorous finite-key analysis for the proposed protocol based on the semidefinite programming (SDP) approach. The numerical results demonstrate that this symmetrically simplified protocol significantly outperforms standard two-dimensional QKD systems in terms of both secret key rate and error tolerance over practical metropolitan distances. Our work explores HD-QKD schemes with fewer devices designed for low-cost deployment and will promote the application of HD-QKD in quantum networks.