Entangled photons are incredibly promising for the fields of quantum computing and communications, but they come with a significant drawback: they vanish after a single use. A recent study introduces a novel method aimed at sustaining communications within a dynamic, unpredictable quantum network. The researchers discovered that by reconstructing these vanished connections, the network eventually stabilizes into a reliable, yet different configuration.
Entangled photons show great potential for quantum computing and communication, yet they have a significant limitation: once utilized, they simply vanish.
In a recent study, physicists from Northwestern University have proposed an innovative approach to uphold communications within an ever-changing quantum network. The team found that by re-establishing the lost connections, the network ultimately adapts and settles into a stable, though altered, condition.
The researchers discovered that the secret lies in incorporating a sufficient number of connections to keep the network operational. However, adding an excessive number of connections can strain resources, while a shortage can lead to a fragmented network that fails to meet users’ needs.
The results from this study could pave the way for the creation of optimally designed quantum networks, enabling ultra-fast computing and highly secure communication.
This research was published today (Jan. 23) in the journal Physical Review Letters.
“Many researchers are dedicating substantial efforts to develop larger and more effective quantum communication networks globally,” stated Northwestern’s István Kovács, the study’s senior author. “However, as soon as a quantum network becomes available to users, it tends to collapse. It’s akin to crossing a bridge and setting it ablaze behind you. Without any intervention, the network disassembles quickly. To address this issue, we created a straightforward model of user behavior. After each communication event, we established a fixed number of links between disconnected nodes. By ensuring there are enough links added after each communication, we preserved network connectivity.”
Kovács, who specializes in complex systems, is an assistant professor of physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences.
Network of vanishing links
Quantum networks function by utilizing quantum entanglement, a phenomenon where two particles remain connected, no matter how far apart they are. Xiangi Meng, an authority on quantum communication and a co-author of the study, refers to entanglement as a “spooky” yet effective resource. At the time of the research, Meng was a research associate in the Kovács group and is now an assistant professor of physics at Rensselaer Polytechnic Institute in New York.
“Quantum entanglement is the eerie, space-defying correlation between quantum particles,” Meng explained. “It serves as a communication link for quantum particles, enabling them to collaborate on complex tasks while ensuring their messages remain secure from eavesdroppers.”
However, when two computers communicate through entangled connections, those links cease to exist after use. The communication itself alters the quantum state of the link, rendering it unusable for future exchanges.
“In traditional communication systems, the infrastructure can handle a vast number of messages,” Kovács remarked. “In a quantum network, each link can transmit only one piece of information before it disintegrates.”
Identifying the critical number
To gain insight into how networks behave amidst constant changes, Kovács and his team developed a simplified model reflecting users within a quantum network. Initially, they allowed users to randomly select others for communication, creating the most efficient communication path and subsequently removing the links along that path. This led to “path percolation,” where the network gradually deteriorates with each communication instance.
After investigating this issue, Kovács and his group sought to propose a solution. Through their modeling, they identified the precise number of links to add after each communication event. This number lies at the critical threshold that supports the network’s integrity while preventing fragmentation. Remarkably, they discovered that the critical number is simply the square root of the total number of users. For instance, if there are one million users, then 1,000 links must be restored for every qubit of information transmitted across the network.
“One might assume that this number grows in proportion to the number of users, or perhaps even at a squared rate since many user pairs can communicate,” Kovács noted. “Our findings revealed that the critical number is a surprisingly small fraction relative to the total number of users. If fewer connections are added, the network collapses and users can no longer communicate.”
Kovács envisions that this information could assist in devising a robust, optimized quantum network capable of withstanding failures. New connections could be automatically created when others disappear, thus enhancing network resilience.
“The classical internet wasn’t originally designed to be completely reliable,” Kovács explained. “It developed organically, influenced by technological limitations and user practices. It wasn’t a conscious design, but rather an emergent phenomenon. Now, with the quantum internet, we have the opportunity to construct a network that fulfills its maximum potential.”
