Quantum Communication Multiplexing in LP-modes Enabled by Photonic Lanterns

The non-cloning theorem of quantum states provides security, but also limits the Secret Key Rate (SKR) for Quantum Key Distribution (QKD) implementations. Multiplexing is a widely used technique to enhance data rates in classical communication systems and can also increase the SKR in QKD systems. Using linearly polarized (LP) modes is an attractive solution as it is compatible with simple fiber designs. This work demonstrates a fiber-based QKD system employing LP mode multiplexing with a photonic lantern to convert the fundamental mode ($LP_{01}$) in separate fibers into higher-order modes ($LP_{11}$) in a single few-mode fiber. The performance of the system is sensitive to polarization dependence, mode alignment, and environmental crosstalk, which requires precise polarization control to minimize Quantum Bit Error Rate (QBER). We report a SKR of 2.34 Mbps over a 24 km 5dB loss fiber link.