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Quantum Computing, Communication, and Simulation

Realization of intermodal fiber/free-space quantum key distribution networks

Francesco Vedovato, Francesco Picciariello, Ilektra Karakosta-Amarantidou, Davide Scalcon, Marco Avesani, Edoardo Rossi, Alessia Scriminich, Giulio Foletto, Matteo Padovan, Alessandro Lorenzetto, Federico Berra, Luca Calderaro, Florian Kutschera, Martin Achleitner, Hannes Hübel, Giuseppe Vallone, Paolo Villoresi

Published Mar 8, 2023• 10 citations
Publisher

Abstract

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.

DOI
10.1117/12.2668341

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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.
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 (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.
Abstract Background Intermodal quantum key distribution enables the full interoperability of fiber networks and free-space channels, which are both necessary elements for the development of a global quantum network. We present a field trial of an intermodal quantum key distribution system in a simple 3-node heterogeneous quantum network — comprised of two polarization-based transmitters and a single receiver — in which the active channel is alternately switched between a free-space link of 620 m and a 17 km-long deployed fiber in the metropolitan area of Padova. Findings The performance of the free-space channel is evaluated against the atmospheric turbulence strength of the link. The field trial lasted for several hours in daylight conditions, attesting the interoperability between fiber and free-space channels, with a secret key rate of the order of kbps for both the channels. Conclusions The quantum key distribution hardware and software require no different strategies to work over the two channels, even if the intrinsic characteristics of the links are clearly different.
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.
We describe systems and methods for the deployment of global quantum key distribution (QKD) networks covering transoceanic, long-haul, metro, and access segments of the network. A comparative study of the state-of-the-art QKD technologies is carried out, including both terrestrial QKD via optical fibers and free-space optics, as well as spaceborne solutions via satellites. We compare the pros and cons of various existing QKD technologies, including channel loss, potential interference, distance, connection topology, deployment cost and requirements, as well as application scenarios. Technical selection criteria and deployment requirements are developed for various different QKD solutions in each segment of networks. For example, optical fiber-based QKD is suitable for access networks due to its limited distance and compatibility with point-to-multipoint (P2MP) topology; with the help of trusted relays, it can be extended to long-haul and metro networks. Spaceborne QKD on the other hand, has much smaller channel loss and extended transmission distance, which can be used for transoceanic and long-haul networks exploiting satellite-based trusted relays.