Seoul National University and University of Seoul team maps complex quantum states like a subway map

A conceptual diagram of the study, which identifies the infinite and complex graph structures that can exist in the interaction between light and a single atom, and proposes a theoretical framework for interpreting them. [Provided by Seoul National University]
A conceptual diagram of the study, which identifies the infinite and complex graph structures that can exist in the interaction between light and a single atom, and proposes a theoretical framework for interpreting them. [Provided by Seoul National University]

A vast network of connections — resembling the neural pathways of the human brain — has been found hidden inside the invisible world of atoms and light. Korean researchers have developed a new method for mapping the intricately entangled quantum states of that world, making them as readable as a subway map.

The National Research Foundation of Korea announced Monday that a research team led by professors Park Nam-kyu and Yoo Sun-kyu of Seoul National University and Professor Park Hyun-hee of the University of Seoul has developed a "magnetic graph" theory that explains the complex interactions between atoms and light within a single unified framework.

The exchange of energy between atoms and light is a core principle underlying quantum computers and advanced optical technologies. The challenge is that when the coupling between the two becomes extremely strong, existing theories struggle to explain it.

Conventional calculations work when the coupling is weak, but approximation methods break down as the interaction moves into "ultrastrong" and "deep-strong" coupling regimes — meaning researchers have had to use different calculation methods depending on coupling strength.

The team found its solution in the concept of a subway map.

The number of photons and the state of an atom are represented as dots — like subway stations — while the probability of transitioning from one state to another is drawn as a connecting line. By encoding the phase information of light into those lines as well, the team converted the complex quantum world into a single vast network.

As a result, even the simplest quantum system of an atom and light produced a "semi-infinite graph" — two types of nodes connected by countless long-range lines. The finding means that even a seemingly simple quantum phenomenon conceals an enormous number of states and connection structures.

The team went a step further, devising a "magnetic Laplacian" index that expresses the complexity of the network as a single number. Applying it allows researchers to track changes in quantum states — from weak coupling all the way to the deep-strong coupling regime that conventional theories struggle to handle — along one continuous numerical scale.

The team also identified why quantum states change so dramatically under strong coupling. After randomly generating and analyzing subnetworks of varying sizes 20,000 times each, they found that the key cause is "phase frustration" — a phenomenon in which light waves traveling through multiple paths fall out of sync and cancel each other out, fundamentally altering the quantum state itself.

Seoul National University professors Park Nam-kyu (left) and Yoo Sun-kyu, and University of Seoul professor Park Hyun-hee. [Provided by the National Research Foundation of Korea]
Seoul National University professors Park Nam-kyu (left) and Yoo Sun-kyu, and University of Seoul professor Park Hyun-hee. [Provided by the National Research Foundation of Korea]

The research could also open a path toward next-generation optical AI development. The approach treats the nodes of the network like "neurons" in an artificial neural network, and the strength and phase of connections between nodes like "synapses." By harnessing the vast number of quantum states within a single atom-light system for computation, the researchers say it may be possible to build a new AI computing architecture distinct from existing photonic neural networks.

However, many hurdles remain before such a system could be realized as actual AI hardware. The team says the technology to stably prepare and measure quantum states must be developed, performance under optical loss and noise must be verified, and the system must be scaled up to incorporate multiple atoms and resonators.

"We have shown that even the simplest quantum phenomenon conceals a vast connection structure within it," said Park Nam-kyu, a professor at Seoul National University. "We will continue research into implementing artificial neural networks by controlling the interaction between a single atom and light."

The findings were published in the international journal Science Advances on Saturday.


nbgkoo@heraldcorp.com