A quantum key distribution network in the metropolitan area of Padova
Marco Avesani, Giulio Foletto, Matteo Padovan, Luca Calderaro, Costantino Agnesi, Elisa Bazzani, Federico Berra, Tommaso Bertapelle, Francesco Picciariello, Francesco B. L. Santagiustina, Davide Scalcon, Alessia Scriminich, Andrea Stanco, Francesco Vedovato, Giuseppe Vallone, Paolo Villoresi
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.
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 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.
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.
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.
Quantum Key Distribution (QKD) has been proven to be an unconditionally secure method for exchanging cryptographic keys under real-world conditions. By exploiting the principles of quantum physics, QKD enables the establishment of secret keys between two communicating parties, ensuring the security of transmitted data. Recently, numerous companies and research institutions have demonstrated the successful implementation of large-scale, high-speed communication systems in both laboratory settings and field tests. Additionally, several advanced network structures have been developed to facilitate secure communication via QKD, contributing to the creation of metropolitan-scale secure networks. In this work, we leverage the state-of-the-art of superconducting nanostrip single-photon detectors (SNSPDs) to enhance the performance of commercial QKD systems. These systems are deployed in a three-node network around the city of Naples, connecting an industrial hub with the Physics Department and the CNR-INO research center.The goal is to establish secure communication channels for exchanging and managing encrypted data. This work represents a significant step toward the practical realization of metropolitan-scale QKD networks and their integration into existing communication infrastructure. By using SNSPDs, we ensure high efficiency and low noise levels, contributing to more reliable and secure key distribution over metropolitan distances. The integration of these technologies demonstrates the feasibility of QKD as a cornerstone for future secure communications in urban environments.
Field trials are of key importance for novel technologies seeking commercialization and widespread adoption. This is also the case for quantum key distribution (QKD), which allows distant parties to distill a secret key with unconditional security. Typically, QKD demonstrations over urban infrastructures require complex stabilization and synchronization systems to maintain a low quantum bit error and high secret key rates over time. Here we present a field trial that exploits low-complexity self-stabilized hardware and a novel synchronization technique, to perform QKD over optical fibers deployed in the city center of Padua, Italy. Two techniques recently introduced by our research group are evaluated in a real-world environment: the iPOGNAC polarization encoder was used for preparation of the quantum states, while temporal synchronization was performed with the Qubit4Sync algorithm. The results here presented demonstrate the validity and robustness of our resource-effective QKD system, which can be easily and rapidly installed in an existing telecommunication infrastructure, thus representing an important step towards mature, efficient, and low-cost QKD systems.
Intermodal quantum key distribution (QKD) enables seamless integration of fiber networks and free-space channels, which are essential to create a global quantum network. Here we report on a recent field trial of an intermodal QKD system within a 3-node heterogeneous quantum network, that we presented in details in the work by F. Picciariello et al., arXiv:2310.17441. The network consists of two polarization-based transmitters and a single receiver, where the active channel is alternately switched between a 620-meter free-space link and a 17-kilometer deployed fiber link in the metropolitan area of Padova. The trial evaluates the performance of the free-space channel in relation to the atmospheric turbulence strength. Conducted over several hours during daylight, the trial confirms the interoperability between fiber and free-space channels, achieving a secret key rate on the order of kbps for both types of channels. Notably, the QKD hardware and software function consistently across both channels without requiring different strategies, despite the distinct characteristics of the links. This switching system offers a cost-effective solution for a trusted QKD network by minimizing the number of devices needed across different networks. The results underscore the feasibility of intermodal QKD systems for future quantum networks.