Enhancements to quantum communication performance utilizing a prototype photonic lantern and multiplexed single-photon detection

The optical interfacing between free-space channels and single-photon detectors plays a crucial role in determining the performance of free-space quantum key distribution (QKD) systems. Direct coupling of free-space signals to detectors often presents significant engineering challenges due to the need for precise alignment and the requirement for adaptive optics, especially when using single-mode fibers. Single-mode fibers offer high spatial resolution but often demand complex adaptive optics to maintain effective coupling, which can reduce system efficiency. Alternatively, multimode fibers (MMFs) are commonly used to address this issue, as they support larger core diameters and are less sensitive to alignment errors. However, larger core sizes introduce modal dispersion, which can limit the bandwidth and, consequently, the achievable secret key rate (SKR). In this work, we demonstrate a prototype photonic lantern (PL) system that enables efficient coupling of free-space multimode light signals into an array of single-mode fibers. This innovative design allows us to preserve the advantages of multimode fibers, such as relaxed alignment requirements and higher coupling efficiency, while mitigating the bandwidth limitations typically associated with large-core fibers. By transitioning from a single multimode fiber to multiple, less multimodal fibers, we reduce the modal dispersion and improve the temporal resolution, effectively enhancing the system's bandwidth and SKR. Our results indicate that this photonic lantern-based approach can significantly optimize the performance of free-space QKD receivers, offering a scalable and efficient solution to overcome key challenges in quantum communication systems.