Overcoming the Ephemeral Nature of Quantum Entanglement

Scientists from Northwestern University have proposed a method to sustain communications in a dynamic, unpredictable quantum network in a new study that was published in the journal Physical Review Letters.

Entangled photons have a significant inherent drawback even though they have enormous potential for quantum communications and computing. They just go away after one use.

The researchers discovered that by reestablishing these vanishing connections, the network eventually stabilizes, albeit in a different way.

Adding the Right Number of Network Connections

The researchers discovered that the secret is to add enough connections to guarantee the network keeps working. An excessive number of connections is expensive and strains available resources. However, if too few connections are added, the network becomes fragmented and unable to meet user demands.

The results may result in quantum networks that are optimally designed for ultra-secure communications and lightning-fast computing.

Many researchers are putting significant efforts into building larger and better quantum communication networks around the globe, but, as soon as a quantum network is opened up to users, it burns down. It is like crossing a bridge and then burning it down behind you. Without intervention, the network quickly dismantles. To tackle this problem, we developed a simple model of users. After each communication event, we added a fixed number of bridges, or links, between disconnected nodes. By adding a large enough number of links after each communication event, we maintained network connectivity.

István Kovács, Study Senior Author and Assistant Professor, Weinberg College of Arts and Sciences, Northwestern University

Each Link can Only Send a Single Piece of Information

Quantum networks operate by utilizing quantum entanglement, a phenomenon where two particles remain connected regardless of the distance between them. One of the study's first authors, Xiangi Meng, a specialist in quantum communication, calls entanglement a “spooky” yet useful tool.

Meng was a Research Associate in the Kovács group at the time of the study.

Quantum entanglement is the spooky, space-time-defying correlation between quantum particles. It is a resource that allows quantum particles to talk to each other, so they can perform complex tasks together while ensuring no eavesdropper can intercept their messages.

Xiangi Meng, Assistant Professor, Rensselaer Polytechnic Institute

However, the links used for communication between two computers vanish when they are entangled. The communication process itself changes the link's quantum state, rendering it useless for additional communications.

In classical communications, the infrastructure has enough capacity to handle many, many messages. In a quantum network, each link can only send a single piece of information. Then it falls apart.

István Kovács, Study Senior Author and Assistant Professor, Weinberg College of Arts and Sciences, Northwestern University

Pinpointing the Magic Number

Kovács and his colleagues constructed a simplified model of users inside a quantum network to gain a better understanding of how networks respond to continuous change. Initially, the researchers gave users the option to choose other users at random to communicate with.

They then eliminated every link along the shortest, most effective communication path between those users. This led to a phenomenon known as “path percolation,” in which each communication event causes the network to progressively fail.

Through modeling, Kovács and his team calculated the exact number of links to include after each communication event. This number represents the critical threshold at which the network can either remain intact or collapse

The team discovered, somewhat surprisingly, that the critical number is simply the square root of the user count. For instance, 1,000 links must be added again for each qubit of data sent over the network if there are one million users.

Kovács said, “It would be natural to expect that this number increases linearly with the number of users, or maybe even quadratically, as the number of user pairs that could communicate. We found the critical number actually is a very small fraction compared to the number of users. But, if you add fewer than that, the network will fall apart, and people cannot communicate.”

According to Kovács, this knowledge might aid in the creation of a reliable, efficient quantum network that is resilient to errors. A more robust network could be created by automatically adding new links when existing ones vanish.

Kovács said, “The classical internet was not built to be fully robust. It naturally emerged due to technological constraints and user behavior. It was not designed, it just happened. But now we can do better with the quantum internet. We can design it to ensure it reaches its full potential.”

Journal Reference:

Meng, X., et al. (2025) Path Percolation in Quantum Communication Networks. Physical Review Letters. doi.org/10.1103/physrevlett.134.030803.