Quantum Key Distribution (QKD) is a cryptography technique that allows two parties to share a secret common key that can be used for a secure exchange of information. To meet the requirements and interests of different final users, the technological development of this technique has rapidly led toward commercial systems for a global quantum infrastructure, namely a quantum network. Communication among parties requires an optical channel, which for short distances is typically provided by optical fiber. However, the limitation of optical losses on longer fiber links, or the unavailability of an exploitable deployed fiber, requires the use of a different QKD medium, i.e. free-space. At the University of Padova we are investigating, both theoretically and experimentally, how Adaptive Optics (AO) can be used to improve the performance of free-space quantum communication, with a particular focus on QKD. Following our theoretical studies on the optimal design for an intermodal free-space/ fiber link, we were able to realize different QKD field trials along a 600m-long free-space link exploiting polarization encoding. Besides the metropolitan area of Padova, we are now working on the development of an extra-urban free-space link, which is more prone to atmospheric turbulence effects. For this link, we equipped a radar tower located on a hill in Teolo with a remotised optical transmitter, and we are now developing the AO system for the receiving telescope, located at the Department of Information Engineering in Padova, with which we plan to realize QKD during day and night time.
Intermodal quantum key distribution (IM-QKD) enables the integration of fiber networks and free-space channels, which are both necessary elements for the development of a global quantum network. IM-QKD permits to extend the reach of free-space links without trusting any additional node, but this requires to efficiently couple the freespace signal into a single-mode fiber (SMF). We present a preliminary point-to-point test conducted in a 620 m free-space channel, with the aim to be used in an intermodal QKD architecture switching between a fiber and a free-space link.
Intermodal quantum key distribution (IM-QKD) enables the integration of fiber networks and free-space connections, which can be ground-to-ground links or involve satellite nodes in orbit. IM-QKD permits to extend the reach of free-space links without trusting any additional node, but this requires to efficiently couple the free-space signal into a single-mode fiber (SMF). We present the implementation of different IM-QKD networks realized in Padova and Vienna, exploiting km-long deployed fibers and free-space channels up to 620 meters. We show that such an intermodal scheme is compatible with both in-house QKD systems and commercially available solutions exploiting polarization encoding at 1550 nm. Remarkably, we realized different QKD tests in daylight and also in rainy conditions.
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.
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