Quantum networks hold the promise of revolutionizing computing and communications by leveraging the phenomenon of quantum entanglement, where particles can instantaneously communicate across vast distances, defying the usual constraints of space and time. This “spooky action at a distance,” as physicist Albert Einstein once called it, enables entangled photons to convey secure information without the risk of interception by potential eavesdroppers.
Despite the immense potential, one fundamental challenge persists in building practical quantum communication networks: entangled photons vanish after just one use. In other words, once a photon has been used to transfer quantum information, it ceases to be entangled and, therefore, cannot be re-used for another round of communication. This makes it incredibly difficult to maintain robust, scalable networks as photon links quickly disappear after each exchange.
Now, in an innovative study published in Physical Review Letters, researchers at Northwestern University have proposed a groundbreaking solution that could potentially keep quantum networks functioning in a constantly evolving and unpredictable environment. Instead of relying solely on the fleeting existence of entangled photons, the scientists suggest a method of rebuilding disappearing connections by adding a specific number of new links. This study reveals how quantum networks could eventually stabilize, balancing the delicate act of maintaining connectivity while optimizing resources.
The Problem with Disappearing Links
Entangled photons are the backbone of quantum communication, serving as the channels that carry vital quantum information between systems, often referred to as qubits. However, this crucial quantum communication tool has an inherent downside: when quantum information is transmitted, the entangled photons used in the process lose their ability to function as entangled links for any future communications.
This “one-use” limitation stands in stark contrast to the way classical communication networks work. For example, in everyday communications such as telephone and internet connections, multiple users can send and receive countless messages through a shared infrastructure that doesn’t degrade with each individual use. In quantum networks, on the other hand, each entangled photon is like a fragile bridge that burns down as soon as it is crossed—usable only once, then gone.
In traditional communication networks, the infrastructure can accommodate the vast flow of data, allowing continuous exchanges without significant interruption. By contrast, in a quantum network, each quantum link can only carry a single piece of information before breaking down, leading to a continual fragmentation of the system. As quantum networks are becoming more scalable and widely considered for real-world applications, the challenge of maintaining these disappearing entangled photons—especially in an increasingly busy network—is becoming more urgent.
A New Strategy for Maintaining Quantum Connectivity
In order to address this challenge, the researchers, led by István Kovács, a professor of physics and astronomy at Northwestern University, have proposed a method that could significantly improve quantum network stability. Their model revolves around the idea of reconnecting disappearing quantum links after each communication event by adding additional quantum connections to the network.
Kovács and his team hypothesized that by rebuilding a minimal set of new links after every communication, they could maintain network connectivity and reduce fragmentation without burdening the system. However, the key challenge was determining how many new links were necessary to ensure the network remained functional but didn’t overload available resources.
The ‘Critical Number’ of Links
Through detailed modeling, Kovács and his team discovered that there is an optimal, critical number of links to add to the network after each communication. This number turned out to be surprisingly small—just the square root of the total number of users within the network.
For instance, in a quantum network with 1 million users, only 1,000 new links need to be added for every quantum communication event. While one might expect this number to be significantly larger as the network grows, the study revealed that a much smaller number of additional links suffices to keep the network connected.
However, adding too many links to the network proved to be inefficient, overwhelming the system with excessive communication resources, which could potentially drain computational or physical capacities. On the other hand, if too few links were re-added, the network quickly fragmented, resulting in communication failures. This delicate balance is crucial for ensuring efficient use of resources while still maintaining connectivity for the entire system.
Kovács compared this problem to building bridges across a river. Without enough bridges, users on the opposite sides of the river are cut off from one another; however, adding too many bridges may increase operational costs and complexity. Hence, the approach hinges on providing just enough support without overcrowding the infrastructure, offering a fine-tuned solution for future quantum communication systems.
Quantum Entanglement: A “Spooky” Resource for Communication
In the world of quantum communication, entanglement is a uniquely valuable phenomenon. It allows quantum particles—typically photons or electrons—to be linked in such a way that their states are instantaneously correlated, regardless of how far apart they are. This means that once entangled, two quantum particles can interact, share information, and even carry out joint operations across vast distances.
The downside, as previously mentioned, is that the entanglement is fragile. Xiangi Meng, a researcher at the time of the study, described entanglement as a “spooky” but effective resource. When quantum particles, such as photons, communicate using entangled links, the process necessarily alters their quantum state, destroying the entanglement in the process and thus rendering the link incapable of carrying any additional information.
“Once an entangled pair is used in a quantum communication event, it can’t be reused,” said Kovács. “This feature is what makes quantum networks so fundamentally different from traditional communication systems.”
In a traditional network, messages can be sent repeatedly along the same route without degrading the system’s performance. Quantum networks, however, are like communication roads with cars that cannot return after each trip, posing a unique challenge.
The Path to Optimizing Quantum Networks
What the Northwestern team’s findings suggest is that, like in classical network infrastructure, quantum networks too can be optimized—not through sheer numbers of links, but through a strategic and calculated approach that adds just the right number of new connections to ensure that the network stays in a viable, connected state.
This research could unlock new capabilities for scaling quantum networks, especially in the domains of quantum computing, quantum cryptography, and secure communications. With faster and more efficient communication links, quantum networks could pave the way for major advancements in fields such as cryptographic security, algorithmic performance, and the transmission of information that’s far more secure than today’s classical systems.
Moreover, Kovács believes that the ability to automatically adapt the network—by restoring vanished links with minimal resource burden—could create a far more resilient quantum communication infrastructure. By engineering quantum networks with these principles in mind, developers will be equipped with a much stronger foundation upon which to build the quantum internet of the future.
“The classical internet emerged through user behavior and technological constraints,” Kovács pointed out. “It wasn’t designed to be resilient. But now, we have the opportunity to build quantum networks from scratch, and with this model, we can ensure they reach their full potential.”
Toward a Resilient Quantum Internet
The ultimate goal of this research is not just maintaining communication in fragile quantum networks, but designing a quantum internet with true fault tolerance—one that can absorb failures without faltering. This advancement in modeling could lead to quantum networks that don’t collapse under strain or malfunction but instead continue to operate under a range of conditions.
While much work remains in creating such a network at the global scale, the model proposed by Kovács and his team is a significant step toward realizing a robust and scalable quantum internet. The ability to rebuild connections and manage network stability could ultimately support the next era of quantum communication technologies, from ultra-secure messaging platforms to advanced quantum supercomputing.
As quantum communication systems move from theoretical exploration to real-world implementation, studies like this are critical in ensuring they function as desired in dynamic, unpredictable environments.
Conclusion
The research led by István Kovács and his team at Northwestern University presents an innovative solution to a key challenge in quantum communication networks: the fragile nature of entangled photon links that disappear after just one use. Through detailed modeling, the researchers discovered that by strategically adding a minimal number of new quantum connections after each communication event, quantum networks can remain stable and functional, avoiding fragmentation while ensuring efficient resource use. This breakthrough could lay the foundation for the development of scalable, resilient quantum networks that support quantum computing and secure communication over long distances.
The model’s findings suggest that the critical number of links needed to maintain network connectivity is surprisingly small, providing an optimal approach to network design. As quantum technologies continue to advance, this work may enable the creation of robust, efficient systems capable of withstanding the dynamic challenges faced by next-generation quantum infrastructures. Moving forward, these principles could help researchers build a quantum internet that functions with minimal interruptions while meeting the demands of a rapidly expanding digital world.
Reference: Xiangyi Meng et al, Path Percolation in Quantum Communication Networks, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.030803. On arXiv: DOI: 10.48550/arxiv.2406.12228