  Quantum key distribution (QKD) protocols based on high-dimensional quantum
states have shown the route to increase the key rate generation while
benefiting of enhanced error tolerance, thus overcoming the limitations of
two-dimensional QKD protocols. Nonetheless, the reliable transmission through
fiber links of high-dimensional quantum states remains an open challenge that
must be addressed to boost their application. Here, we demonstrate the reliable
transmission over a 2 km long multicore fiber of path-encoded high-dimensional
quantum states. Leveraging on a phase-locked loop system, a stable
interferometric detection is guaranteed, allowing for low error rates and the
generation of 6.3 Mbit/s of secret key rate.
