Quantum key distribution (QKD) is an advanced technology that provides ultimate secure communication by exploiting quantum states of light as information carriers. In early QKD protocols, each bit of the key was encoded using a two-dimensional quantum. Advanced QKD protocols are based on encoding the key in d-dimensional quantum states, called qudits. The higher information capacity of qudits allows a higher secure key rate and improves the robustness of the key distribution protocols to the noise, leading to higher threshold values for the quantum bit error rate (QBER). However, so far the proposed high-dimensional protocols required complex experimental resources, thus raising the cost of practical high-dimensional systems and limiting their use.

We have recently analyzed and demonstrated a novel scheme for fiber-based QKD protocol in an arbitrary dimension, based on the most popular commercial hardware for binary time-bin encoding. We demonstrated high-dimensional key distribution over 40 km single-mode fiber. We observed a two-fold enhancement of the secret key rate in comparison to the two-dimensional Coherent One Way (COW) protocol, without introducing any hardware modifications to the two-dimensional system. This work holds great potential to enhance the performance of already installed QKD systems by software updates alone.