This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A new graph-theoretic framework provides a unified description of atom-light interactions across regimes ranging from weak to deep-strong coupling. A research team comprising professors Sunkyu Yu and Namkyoo Park of the Department of Electrical and Computer Engineering at Seoul National University College of Engineering and professor Xianji Piao of the University of Seoul has discovered that even the interaction between a single atom and light—one of the simplest settings in quantum physics—contains an enormous, intricately connected network.
The study was published Sept. 25 in the journal Science Advances. By developing a mathematical framework that represents this hidden structure as a subway map, the researchers established a unified way to understand atom-light interactions across coupling regimes that previously required different theoretical approximations. The findings could open new possibilities for quantum computing and photonic neural networks.
The National Research Foundation of Korea (NRF), chaired by Won-Hwa Hong, announced that a research team led by Park and Yu of Seoul National University and Piao of the University of Seoul has developed a "magnetic graph" theory that quantifies atom-light interactions using a single measure. Interactions between an atom and light—theoretically described by the quantum Rabi model, the most fundamental model of atom-light interactions—are a building block of quantum computing and optical technologies. When an atom absorbs a photon, its energy state changes; when it releases energy, it may emit a photon.
Repeated interactions of this kind underpin quantum information processing, quantum sensing, lasers, nonlinear optics and numerous other light-matter technologies. When the interaction is sufficiently weak, its behavior can be described accurately using well-established approximations. However, as the coupling between the atom and light becomes extremely strong—entering the ultrastrong or deep-strong coupling regimes—these conventional approximations begin to fail.
Ultrastrong and deep-strong coupling are regimes in which the strength of the atom-light interaction becomes comparable to or greater than the intrinsic frequency of light. Under these conditions, the standard approximations applied to the quantum Rabi model are no longer reliable. As a result, researchers have traditionally used different approximation methods to describe different coupling regimes.
No single conventional theory can fully describe behavior across all regimes. A new mathematical language was therefore needed—one capable of representing complex quantum states at a glance and describing the entire evolution from weak to deep-strong coupling within a unified framework. Inspired by graph theory—which can be used to interpret subway maps, brain architecture and social networks within a single framework—the research team devised a method for translating the quantum world into a graph.
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