In the rapidly evolving domain of quantum communications, the ability to maintain optical coherence over long distances has emerged as a vital ingredient for constructing the quantum internet of the future. Recent groundbreaking work by Pittaluga and colleagues marks a significant leap forward in this area, demonstrating coherent quantum communication over an unprecedented 254-kilometer span of deployed commercial fiber optic infrastructure connecting Frankfurt and Kehl in Germany. This achievement not only sets a new benchmark for distance in quantum key distribution (QKD) but also showcases the potential for integrating advanced quantum protocols within existing telecommunications frameworks without reliance on bulky cryogenic technology.
At the heart of the quantum internet vision lies the exploitation of quantum coherence, a delicate property that enables secure transmission and processing of quantum information by preserving the phase relationship between quantum states. Traditional approaches to realizing coherence-preserving quantum networks have been hindered by the considerable technical challenges associated with ultra-stable optical cavities and the need for cryogenically cooled single-photon detectors. These requirements, although effective at laboratory scale, have limited scalability and posed obstacles for deployment in real-world networking environments spanning urban and intercity distances.
The novel experimental implementation presented by Pittaluga et al. advances the field by utilizing a coherence-based twin-field QKD protocol specifically engineered to operate across a commercial telecom network infrastructure. This protocol, renowned for its superior performance over long distances, encodes quantum information onto the phase of single photons traveled through optical fibers, leveraging the principle that quantum signals encoded as phase references can exceed traditional QKD’s distance limit. The team’s success in maintaining phase coherence across an operational dark fiber link of 254 kilometers—more than doubling previous practical QKD transmission distances—marks a milestone that redefines the terms for deploying quantum-secured communication systems on metropolitan and regional scales.
A pivotal component of the experiment’s breakthrough was the development of a scalable architecture to distribute and stabilize optical coherence across this vast fiber span. Unlike prior setups dependent on cryogenically cooled detectors operating at near absolute zero, the researchers achieved reliable single-photon detection with non-cryogenic detectors combined with an off-band phase stabilization method. This approach actively compensates for fluctuations in the optical path length caused by environmental disturbances such as temperature variations and mechanical vibrations, which typically disrupt the fragile coherent signals. By integrating these techniques, the setup attains a level of phase stability necessary to enable practical quantum key generation rates compatible with encryption applications.
The experimental results reveal an impressive secure key generation rate of approximately 110 bits per second—an enormous feat given the network’s urban reach and the technical challenges involved. This throughput signifies that long-distance quantum communication protocols can be effectively realized with currently available telecom components, bridging the gap between laboratory research and real-world quantum networks. Moreover, the reported network embodies measurement-device-independent (MDI) properties, a feature that fundamentally immunizes the system against a broad array of detection-based hacking attacks, thus reinforcing the practical security claims of quantum key distribution.
Equally consequential is the implication of this work for the evolving landscape of quantum repeaters and quantum networking infrastructures. Repeater nodes are crucial in extending the reach of quantum information transfer by mitigating losses and decoherence over long fiber spans. The demonstration of repeater-like behavior in a deployed fiber network indicates the possibility of scalable, modular quantum communication links that maintain integrity and security without incurring prohibitive hardware complexity or maintenance demands. Such networks could ultimately serve as the backbone for a global quantum internet enabling applications ranging from unconditionally secure communications to distributed quantum sensing and computing.
This ability to harness and distribute optical coherence over existing submarine and terrestrial telecom fiber architectures aligns well with industry demand for practical quantum networking solutions that can be incrementally integrated into present-day infrastructures. By circumventing the need for cryogenic cooling and leveraging commercially available components, the technique offers a cost-effective and energy-efficient pathway for providers to adopt quantum-secure communication services without overhauling their fundamental network designs. This compatibility could accelerate the transition from proof-of-concept experiments to robust commercial quantum networks.
Furthermore, the work by Pittaluga et al. addresses one of the most persistent challenges in scalable quantum communication: overcoming the decoherence processes introduced by long fiber channels subjected to temperature gradients, mechanical stress, and environmental perturbations. The off-band phase stabilization approach not only compensates for these disturbances but does so in a way that preserves the delicate quantum states crucial to twin-field QKD protocols. This advancement illustrates that coherent quantum information can be reliably transmitted in real operational conditions, rather than under tightly controlled laboratory environments.
The findings also open exciting prospects for future multi-node quantum networks, where several quantum processors or sensors are linked over metropolitan or regional areas. As coherence distribution scales up, complex quantum communication protocols such as entanglement swapping, teleportation between non-adjacent nodes, and distributed quantum computation become achievable with increased robustness. This capability is critical for realizing a fully operational quantum internet that connects distant quantum devices while maintaining high fidelity and security.
Additionally, the long-distance coherent quantum communication technique is anticipated to have significant implications for quantum sensing networks that require synchronized phase referencing over extended geographical scales. Accurate phase coherence ensures enhanced sensitivity and precision in measurements, fostering advancements in fields ranging from fundamental physics tests to navigation and environmental monitoring. The technology demonstrated may therefore catalyze interdisciplinary innovation leveraging quantum-enhanced measurement capabilities.
It is important to emphasize that this work exemplifies a synergy between quantum physics and classical telecommunication engineering. The adaptation of quantum protocols to conventional fiber optic channels—complete with their practical limitations such as attenuation, dispersion, and noise—underlines the crucial interplay between theoretical quantum advances and system-level engineering solutions. This confluence accelerates the translation of foundational quantum science into impactful technological innovations with tangible societal benefits.
In essence, the demonstration of scalability, security, and operational practicality in this quantum network experiment encapsulates a pivotal moment in the field. As quantum communications inch closer to mainstream adoption, these results provide a compelling blueprint for how coherence-based quantum protocols can be harnessed over real-world fiber optic infrastructures. The successful realization of such high-performance, long-distance quantum links heralds a new era in cybersecurity and quantum information science poised to redefine the future of networked communications globally.
In summary, Pittaluga and colleagues’ elegant integration of coherent quantum communication protocols with deployed commercial telecom fibers without reliance on cryogenics points the way toward a practical quantum internet. Their results extend the achievable distance for QKD and establish a scalable framework that can accommodate future advances in quantum repeaters, sensing, and distributed computation. By bridging the gap between theoretical protocols and operational networks, this research lays the foundation for a secure, high-speed quantum communication era embedded within the global telecommunications landscape.
Subject of Research: Long-distance coherent quantum communications and quantum key distribution over deployed commercial telecommunications networks.
Article Title: Long-distance coherent quantum communications in deployed telecom networks.
Article References:
Pittaluga, M., Lo, Y.S., Brzosko, A. et al. Long-distance coherent quantum communications in deployed telecom networks. Nature 640, 911–917 (2025). https://doi.org/10.1038/s41586-025-08801-w
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