Schematic of a quantum communication network, where shared entangled links allow multiple users at one location to connect with users at another location, routed along possibly different paths. Credit: Shao et al

Quantum technologies, systems that process, transfer or store information leveraging quantum mechanical effects, could tackle some real-world problems faster and more effectively than their classical counterparts. In recent years, some engineers have been focusing their efforts on the development of quantum communication systems, which could eventually enable the creation of a "quantum internet" (i.e., an equivalent of the internet in which information is shared via quantum physical effects).

Networks of quantum devices are typically established leveraging quantum entanglement, a correlation that ensures that the state of one particle or system instantly relates to the state of another distant particle or system. A key assumption in the field of quantum science is that greater entanglement would be linked to more reliable communications.

Researchers at Northwestern University recently published a paper in Physical Review Letters that challenges this assumption, showing that, in some realistic scenarios, more entanglement can adversely impact the quality of communications. Their study could inform efforts aimed at building reliable quantum communication networks, potentially also contributing to the future design of a quantum internet.

"Most work on quantum communication networks has focused on communication between just two users at a time," Adilson Motter, senior author of the paper, told Phys.org. "We wanted to understand what changes when many pairs of users try to communicate simultaneously.

"Since we don’t yet know what a future quantum internet will look like, we took inspiration from today’s large-scale communication and traffic networks, which tend to operate in a decentralized way, with users competing for shared resources."

Modeling a quantum communication network

Drawing inspiration from the underpinnings of today’s communication networks, Motter and his colleagues created a model of a quantum communication network. A quantum communication network entails the transfer of information that is encoded in so-called quantum states, which are generally shared between distant entangled devices.

"The quality of this entanglement is quantified by its fidelity, which measures how close the shared state is to a maximally entangled one—the higher the fidelity, the better," explained Motter. "In the non-cooperative protocols we studied, each user pair independently chooses an available path of entangled links through the network to maximize its own end-to-end fidelity. We analyzed these choices and calculated the resulting fidelities to assess the overall performance of the protocols."

Surprisingly, the researchers’ analyses revealed that when users in a quantum communication network act selfishly—meaning that they only try to optimize their own end-to-end entanglement—the overall quality of the resulting entanglement can decline when more entanglement is initially available. This could, in turn, adversely impact communications, limiting the performance of large-scale quantum networks.

Guiding the future development of a ‘quantum internet’

This recent study identifies an unexpected limitation of quantum communication networks, showing that adding more entanglement does not necessarily improve a network’s performance. It shows that this would specifically happen if users acted noncooperatively, as they are expected to act in the context of a decentralized quantum internet.

"The effect we observed stems from the presence of mixed entangled states, which arise inevitably in practice due to noise and other uncontrolled interactions with the environment," said Yanxuan Shao, first author of the paper. "We show that when such states are present, individually reasonable routing choices generate a quantum analog of ‘selfish routing,’ which can drive the system to lower performance as the entanglement budget grows.

"The result overturns the widely held assumption that more entanglement is always better, revealing fundamental trade-offs among efficiency, fairness, and decentralization in multi-user quantum communication."

The effect uncovered by the researchers is similar to what happens in some instances when drivers behave "selfishly" on the road, creating congestion by taking a route that is best for them without considering other drivers. Shao, Motter and their colleagues identified scenarios in which this paradoxical effect would emerge in quantum networks and showed that it becomes more pronounced in larger networks.

"Somewhat counterintuitively, the network can perform better when certain entangled links are removed, resulting in higher overall communication quality," said Motter. "This is analogous to a phenomenon previously identified in classical traffic networks, known as the Braess paradox, where the removal of a road can lead to reduced congestion and improved average travel time."

The initial results gathered by this research team expose a key challenge that could prevent the efficient implementation of large quantum communication networks. Future work could further investigate the effect uncovered by the authors and propose possible strategies to prevent or mitigate it.

"Going forward, we’re interested in identifying conditions under which decentralized quantum communication protocols can both use the available entangled links efficiently and remain fair, enabling different user pairs to achieve comparable end-to-end fidelities," added Motter.

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Publication details

Yanxuan Shao et al, Noncooperative Quantum Networks, Physical Review Letters (2025). DOI: 10.1103/253d-s68r. On arXiv: DOI: 10.48550/arxiv.2512.15884

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Citation: Too much entanglement? Quantum networks can suffer from ‘selfish routing,’ study shows (2026, January 21) retrieved 21 January 2026 from https://phys.org/news/2026-01-entanglement-quantum-networks-selfish-routing.html

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