Quantum key distribution implemented with d-level time-bin entangled photons

High-dimensional photon states (qudits) are pivotal to enhance the information capacity, noise robustness, and data rates of quantum communications. Time-bin entangled qudits are promising candidates for implementing high-dimensional quantum communications over optical fiber networks with processing rates approaching those of classical telecommunications. However, their use is hindered by phase instability, timing inaccuracy, and low scalability of interferometric schemes needed for time-bin processing. As well, increasing the number of time bins per photon state typically requires decreasing the repetition rate of the system, affecting in turn the effective qudit rates. Here, we demonstrate a fiber-pigtailed, integrated photonic platform enabling the generation and processing of picosecond-spaced time-bin entangled qudits in the telecommunication C band via an on-chip interferometry system. We experimentally demonstrate the Bennett-Brassard-Mermin 1992 quantum key distribution protocol with time-bin entangled qudits and extend it over a 60 km-long optical fiber link, by showing dimensionality scaling without sacrificing the repetition rate. Our approach enables the manipulation of time-bin entangled qudits at processing speeds typical of standard telecommunications (10 s of GHz) with high quantum information capacity per single frequency channel, representing an important step towards an efficient implementation of high-data rate quantum communications in standard, multi-user optical fiber networks.