Multi-Channel Time-Bin Quantum Key Distribution over a 70-km-Long Deployed Fiber Link

Quantum key distribution (QKD), in which security is ensured by physical quantum laws, is shown as the most secure communication between two distant users [1]–[3]. Recent efforts in global research focus on providing the infrastructure that can support QKD and extending applications to large scale of users and distances. Three types of links are explored up to date: terrestrial free-space links (limited by visible distance), fiber-based links (repeater-free range limited to a few 100 km [1]) and finally satellite links, which allow to bridge intercontinental distances. For each of these scenarios, different types of hardware are involved and state encoding (polarization-, time- [1]–[2], spatial- and frequency-based [3]) is tailored to mitigate link impairments. Additionally, the effort towards higher QKD rates is given [1]–[3] that future global quantum communication networks support a wider range of use cases.