One of the most advanced technologies within the field of quantum mechanics is quantum key distribution (QKD), which allows the secure generation of secret keys among remote users. In order for QKD to be more widely adopted, it must be integrated into existing classical communication systems. However, this can be difficult due to the use of various technologies and channels in deployed networks. Recently, we developed a QKD network in the metropolitan area of Padova, which connects various nodes across the city through a combination of fiber and free-space links. By utilizing a modular design based on the iPOGNAC encoder and the Qubit4Sync method, we have realized portable and adaptable systems that operate in the C and O bands. This allowed us to deploy and test the compatibility of both research and commercial QKD systems by ThinkQuantum with classical communication over a variety of links, as well as their ability to switch between free-space and fiber connections. Finally, we developed and experimentally implemented complex network configurations such as star networks, where a fiber-based transmitter and free-space transmitter could operate with a single receiver.
Quantum key distribution (QKD) is one of the most mature among the quantum technologies that allows two remote users to generate secret keys with unconditional security. To increase its adoption, simple, low-cost, and robust systems are necessary, together with demonstrations in real environments. Here, we present a QKD field-trial over optical fibers deployed in the city center of Padua, Italy. Our system exploits two key technologies developed by our group: a low-error, self-stabilized polarization encoder, called iPOGNAC, and a novel synchronization technique, called Qubit4Sync, which allows us to minimize the experimental complexity of our system.
The future global-scale quantum communication network will require free-space and satellite links able to work in daylight conditions and compatible with the telecom fiber infrastructure. Here we present a full prototype for daylight quantum key distribution at 1550 nm exploiting an integrated silicon-photonics chip as state encoder. We tested our prototype in the urban area of Padua (Italy) over a 145m-long free-space link, obtaining a quantum bit error rate around 0.5% and an averaged secret key rate of 30 kbps. The developed chip represents a cost-effective solution for portable free-space transmitters and a promising resource for future satellite missions.
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