Connecting quantum cities across Europe

French researchers have designed a simulation for a large-scale, satellite-based quantum network across Europe called “The Qloud”. The innovative architecture aims to connect metropolitan quantum networks with minimal hardware requirements for the end-users or “Qlients”. In a recent study published in the New Journal of Physics, they used a quantum network simulator to recreate “Quantum Cities”, focusing on quantum key distribution (QKD) rates between distant places in Europe using realistic technology.

The Qloud architecture features a star topology within Quantum Cities. A powerful central node, the “Qonnector,” connects to each Qlient through optical fibers. This setup allows for centralized routing and asymmetrical distribution of hardware, with the Qonnector serving as a hub for quantum services. They can also receive and process photonic states from satellites. Qlients, representing end-users, are equipped with limited quantum hardware, capable of manipulating one qubit at a time.

Satellites act as backbone nodes between Qonnectors, facilitating communication between Quantum Cities. The researchers simulated satellite-to-ground links, incorporating realistic noise models to account for atmospheric perturbations, particularly atmospheric absorption and beam-wandering effects. The simulations used real satellite data, allowing for the tracking of satellite orbits and the calculation of elevation and distance to ground stations.

The researchers studied four different satellite orbits: the QSS (Micius) orbit, the Starlink-1013 orbit, the Iridium-113 orbit, and the Cosmos-2545 orbit. Initial simulations have focused on a simple downlink scenario to determine the best satellite for quantum communication. The Micius and Starlink satellites, both LEO satellites, have shown the highest rates of successful photon transmission to the ground station in Paris. The study highlights the inherent trade-off in satellite communication between the distance of the satellite to Earth and the time frame during which it can be used.

The publication also looks into the influence of different parameters on the QKD rate. They found that the aperture radius of the receiving telescope, the beam waist divergence, the pointing error, and the atmospheric aerosol model significantly impact the transmission of photons. For instance, increasing the aperture radius of the receiving telescope resulted in a substantial improvement in the number of successfully measured qubits, suggesting a key area for future technological advancements.

The researchers simulated two scenarios to assess the practicality of satellite-based QKD between Quantum Cities. In the trusted satellite scenario, the satellite establishes secret keys with the Qonnectors of both cities using the BB84 protocol. Then, Qlients establish secret keys with their respective Qonnectors. The Qonnectors securely relay the keys between the two Qlients via the satellite. This scenario relies on the trustworthiness of all intermediary nodes. The untrusted satellite scenario utilizes the BBM92 protocol, an entanglement-based QKD method. Here, the satellite generates entangled EPR pairs, sending one qubit to each Qonnector. The Qonnectors then couple the received qubits into optical fibers and transmit them to their designated Qlients. This approach eliminates the need to trust the satellite.

While the simulations demonstrated the feasibility of inter-city QKD, they also highlighted the limitations caused by the limited visibility window of a single satellite. To address this, the researchers suggest the use of multiple satellite passes over several days to accumulate key material or using constellations of satellites for continuous key generation.

However, these simulations do not cover all complexities in a real quantum network. There are many factors like variations in atmospheric turbulence, operational wavelength, and synchronization issues that need to be further investigated. They also discuss briefly the potential of high-altitude balloons as an alternative to satellites. Follow-up preprint studies by some of the researchers have looked into a detailed atmospheric model and use of adaptive optics in the entanglement-based scenario, arXiv.2411.09564, and a detailed study of trade-offs involved in the high-altitude balloon scenario, arXiv.2412.03356.

Overall, this article shows great prospects for satellite-based quantum communications for connecting Quantum Cities across Europe, showcasing The promise of the the Qloud architecture for emerging quantum networking applications.