High-dimensional quantum key distribution using a multi-plane light converter

High-dimensional quantum key distribution (QKD) offers higher information capacity and stronger resilience to noise compared to its binary counterpart. However, these advantages are often hindered by the difficulty of realizing the required high-dimensional measurements and transformations. Here, we implement a large-scale multi-plane light converter (MPLC) and program it as a high-dimensional mode sorter of spatial modes for QKD. We use the MPLC to demonstrate five-dimensional QKD using six mutually unbiased bases and 25-dimensional QKD with two mutually unbiased bases in the same experimental setup. We further propose a construction of pairs of mutually unbiased bases that are robust to experimental errors, and their measurement complexity scales only with the square root of the encoded dimension, paving the way for QKD implementations in higher dimensions.