Towards metropolitan free-space quantum networks

The development of a quantum internet is a significant goal, with quantum key distribution (QKD) networks currently leading the way. While fiber-based systems are suitable for metropolitan areas, their reliance on existing infrastructure presents limitations. This paper explores the potential of free-space entanglement-based quantum networks as a more flexible and readily deployable alternative for metropolitan applications. Current research focuses on extending the range of quantum links (via satellites and fiber-based quantum repeaters) and improving scalability and accessibility of metropolitan networks (mostly relying on fiber links). However, in many scenarios, fiber connections are not feasible. Terrestrial free-space links offer a potential solution, but face challenges such as atmospheric turbulence and daylight noise. While entanglement-based QKD has been explored over free-space links, its practical applicability for readily deployable metropolitan networks has been questioned due to limited demonstrated key rates, particularly in daylight conditions. This research aims to address these concerns by proposing and demonstrating a metropolitan free-space network architecture based on entanglement distribution, providing a solution for secure communication in diverse environments where fiber links are unavailable.

The paper reviews existing literature on quantum communication networks, highlighting the advancements in fiber-based QKD and the challenges in scalability and deployment. It discusses the limitations of relying solely on fiber links in metropolitan areas and the potential of free-space links as a complementary technology. The authors cite previous work on entanglement-based free-space QKD, noting the relatively low key rates achieved in previous daylight experiments (hundreds of bps over short distances). This lack of high key rate performance in daylight conditions has been a significant argument against the practicality of entanglement-based free-space QKD for metropolitan applications.

The researchers developed a deployable free-space QKD system suitable for metropolitan environments. The system features a portable entangled photon pair source, deployable free-space terminals, and compact, passive quantum state analysis and detection modules optimized for daytime operation. The entanglement server (ES) generates pairs of polarization-entangled photons at 810 nm; one photon is sent to Alice via fiber, the other to Bob via free-space. A 1064-nm beacon laser co-propagates with the signal for beam stabilization using a closed-loop system with fast-steering mirrors. Both Alice and Bob utilize quantum receiver subsystems (QRS) implementing the BBM92 protocol. Bob's QRS incorporates spectral and spatial filtering for daylight operation. The classical communication channel is implemented with radio antennas. The system's performance was evaluated through QKD experiments over a 1.7-km link (between Fraunhofer IOF and a container on a building) and a 300-m link (between government offices), assessing key rates under nighttime and daylight conditions. Key rate calculations considered finite key effects and the security proof from Tomamichel and Leverrier (2017). Extrapolations to longer links (10 km) and multi-user scenarios (using wavelength division multiplexing) were also performed based on the experimental data and models for beam spreading due to atmospheric turbulence. The paper details the characteristics of the entangled photon source, the quantum receiver subsystems (including daylight filtering methods), the design and performance of the free-space terminals, and the post-processing steps (including time synchronization, sifting, error correction, and privacy amplification).

The researchers demonstrated high secure key rates with their deployable free-space QKD system. Specifically, they achieved up to 5.7 kbps over a 1.7-km link at night and maintained key rates above 2.5 kbps even in direct sunlight. The experiments showed a clear correlation between solar radiation levels and system performance, with higher background noise in direct sunlight. However, even under these challenging conditions, the system maintained robust performance due to temporal filtering capabilities of the system leveraging photon pair correlations. Extrapolations based on the nighttime experiment suggest that 3.3 kbps key rates are achievable over a 10-km link, and over 1.1 kbps for a dual 1.7-km free-space link scenario at night. For multi-user scenarios, estimates suggest kbps nighttime rates and hundreds of bps in daytime, considering multiple 1.7-km free-space links and wavelength division multiplexing. This represents a significant improvement compared to previous entanglement-based free-space QKD daylight experiments in both secret key rate and link range.

The results demonstrate the feasibility of high-rate entanglement-based free-space QKD over extended distances, even under challenging daylight conditions. The achieved key rates are competitive with those of prepare-and-measure systems, and the system's deployability makes it suitable for various metropolitan applications where fiber connections may be impractical. The use of 810 nm for the quantum channel offers flexibility, potentially acting as an interface between free-space and fiber segments in a heterogeneous network. The architecture's scalability through the addition of multiple entanglement servers and the potential for incorporating entanglement swapping further enhance its potential. The findings address the previous skepticism about the feasibility of entanglement-based free-space QKD for metropolitan-scale networks.

This work successfully demonstrates the potential of entanglement-based free-space quantum networks for metropolitan applications. The developed system, with its high key rates and ease of deployment, offers a practical and efficient alternative to fiber-based solutions in scenarios with limited fiber infrastructure. Future improvements could include extending the system to fully connected multi-user networks, using adaptive optics to enhance daylight performance, and exploiting higher-dimensional entanglement for increased information capacity.

The study primarily focuses on point-to-point links, with extrapolations for multi-user scenarios. The impact of atmospheric conditions (beyond turbulence and solar radiation) on long-term system performance was not extensively evaluated. While extrapolations were made to longer distances and multi-user settings, these remain theoretical estimates until experimentally verified. Optimization of filtering parameters (spectral and temporal) in daytime conditions could lead to further improvement in key rates. The reliance on a central entanglement server implies a single point of failure, although mitigation strategies, such as redundancy and backup systems, could address this.