A scalable multi-partite high-dimensional quantum network for high-rate entanglement distribution and secure communications

Using wavelength and time multiplexing for quantum key distribution networks to serve multiple users over a single link received tremendous interest since it enables using a single link to communicate with multiple parties with minimal equipment. However, most of the recent implementations are suffering from rate limitations due to the usage of qubits carrying binary information, susceptibility against error, and noise with low source and detector efficiencies. Here, we have demonstrated a four-user network that uses high-dimensional arrival time bin encoding that can exceed state-of-the-art implementations in performance and work under high background noise. To overcome these limitations to achieve scalable quantum key distribution networks, we employed high-dimensional entanglement on an energy-time basis to encode multiple bits of information per entangled photon pair shared between two users. Furthermore, we employ nonlocal dispersion compensation modules to reduce the timing errors acquired during propagation over long distances and to monitor channel security using time and energy correlations.

Compared to similar works using wavelength and time, we obtained a gain in secure key rates over an order of magnitude to two orders of magnitude, up to 16 kbits/s at the source and 3.2 kbits/s at 21 km distance. We achieved a non-binary photon information efficiency of up to 2.137 bits/pair while withstanding a quantum bit error rate of 30%, where the binary protocols are susceptible to bit error rates above 10%. Using the dispersive basis for security measurements, we obtained a Holevo leakage only up to 0.3507 bits/pair. The protocol is robust against noisy channels, while it offers full and simultaneous connectivity among the users using a central entanglement provider. The network can be implemented with commercial telecommunications equipment where each user only needs a single channel and detector to be connected, reducing the cost further. Furthermore, the QKD implementation is also a figure of merit for other quantum communication tasks and can be used for high-fidelity entanglement distribution, superdense coding, teleportation, or distributed quantum computing.