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Ryusuke Hamazaki’s Search for Common Structures

Wind blows. Rivers flow. Chemical reactions change color. Living organisms evolve, while ecosystems change through countless interactions between species. The world around us is full of complex phenomena that are constantly changing.

“Many of these phenomena are very familiar to us,” says Ryusuke Hamazaki, a theoretical physicist at the RIKEN Pioneering Research Institute (PRI). “But from the perspective of physics, there is still an enormous amount that we do not understand.”  

Hamazaki studies nonequilibrium quantum statistical mechanics. At the heart of his research is a question that connects many of the problems he explores: can seemingly different phenomena share a common underlying structure?

At first glance, quantum systems and the evolution of living organisms may have little in common. Yet when described in the right mathematical language, similar patterns can sometimes emerge.

“Finding that completely different things actually share a common structure is something I find very interesting.”

For Hamazaki, the diversity of complex phenomena is precisely what makes them worth exploring. Even if no single law governs them all, he wants to understand whether common structures can be found beneath their differences.

From Mathematics to a New Direction

Hamazaki’s interest in science began with mathematics.

“I liked arithmetic and mathematics from elementary school. By the time I was in junior high school, I was even making up problems and trying to solve them myself.”

By high school, his interest had begun to expand from mathematics to physics. One important encounter was The Feynman Lectures on Physics, which he found in a library.

“It was not simply a book explaining physics. It conveyed what was interesting about physics itself. After reading it, I wanted to study physics more seriously.”

His growing interest eventually led him to the International Physics Olympiad, where he represented Japan and won a silver medal. The competition, held in Croatia, was also his first trip abroad.

“To be honest, I studied partly because I really wanted to go to Croatia,” he says with a laugh.

At the University of Tokyo, Hamazaki continued to study physics, eventually joining Professor Masahito Ueda’s laboratory as a graduate student. At first, he wanted to work on Maxwell’s demon, a thought experiment involving measurement and feedback that raises fundamental questions about the second law of thermodynamics. The topic was closely related to the research he would later pursue, but much of the groundwork had already been laid.

“I had just joined the laboratory, so I did not really know that at the time,” he recalls.

During this period, Ueda asked him a basic question related to a more fundamental issue: if you pick a quantum state at random, does it represent a state of thermal equilibrium?

Hamazaki did not know the answer.

The answer is yes, but the question pointed toward a broader issue that would become the focus of his master’s and doctoral research: why can the laws of statistical mechanics emerge from quantum mechanics?

“Professor Ueda asked me that question, and I didn’t know the answer. That led me to think about why statistical mechanics can be derived from quantum mechanics.”

Rather than simply determining a specific research topic for him, the question helped set the direction of his research. Hamazaki went on to explore this problem throughout his graduate studies, laying the foundation for the research he conducts today.

“The fact that I am working on what I do now is really thanks to Professor Ueda’s guidance.”

For Hamazaki, that question did not simply provide an answer. It helped set the direction of the research that followed.

When Different Worlds Share the Same Structure

One of the central themes of Hamazaki’s research is nonequilibrium physics. Nonequilibrium phenomena are extraordinarily diverse. Energy and matter flow, conditions change over time, and many elements interact to produce behavior that cannot be understood simply by looking at each component separately.

“Nonequilibrium phenomena are incredibly diverse,” Hamazaki says. “So if you ask whether there is one universal law that governs all nonequilibrium systems, there may not even be one.”

Yet common structures can sometimes emerge. That possibility is what draws Hamazaki in.

“When you find that different phenomena share a common structure, that is very exciting. Then you want to know whether the connection is really universal or whether it is just a coincidence.”

One example comes from his work on evolutionary and ecological dynamics. Evolution and ecosystems involve processes such as mutation, natural selection, and competition. Quantum physics may seem far removed from such processes. Yet when these systems are described mathematically, similar structures can emerge. In Hamazaki’s work^[1], ideas connected to statistical physics and information theory have helped reveal unexpected links between quantum dynamics and evolutionary or ecological processes. For Hamazaki, these connections are particularly compelling.

“Things that seem completely unrelated can actually have the same underlying structure. Finding that is very intellectually stimulating.”

Searching for universality, however, does not mean reducing a complex world to a simple one. Instead, it means asking whether certain structures appear repeatedly—even in systems that, on the surface, seem to have nothing in common.

Reading Physics from Fluctuations

One of Hamazaki’s recent research interests concerns an unusual feature of quantum systems: measurement does not simply reveal what is already there. It can change the system itself.

In everyday life, measurement is usually thought of as a way of observing a system. In the quantum world, however, the act of measurement can influence the system being measured. This question has become an important theme in Hamazaki’s recent work.

In 2025, Hamazaki and his collaborators discovered a new type of spectral phase transition in monitored quantum systems, uncovering mathematical features that connect measurement-induced phase transitions with more conventional forms of phase transition^[2].

“Recently, we have also started to see common mathematical structures between different types of phase transitions.”

For Hamazaki, such connections raise a broader question: why should apparently different phenomena share the same structure? And how far can such similarities be extended?

Then, in 2026, they turned their attention to the measurement record itself. In a theoretical study of a continuously monitored quantum many-body system, the researchers investigated fluctuations in the number of detected atoms. They found that as the strength of measurement increases, the system can exhibit unusually large fluctuations—signatures of collective many-body effects induced by measurement backaction. In other words, the record produced by the measurement may itself contain clues about how measurement is changing the physics of the system.

The idea is captured in the graphical abstract below, which illustrates a continuously monitored quantum many-body system and the fluctuations in the number of detected atoms. As the measurement becomes stronger, the fluctuations change from ordinary to anomalously large—offering a way to read signatures of measurement-induced many-body physics directly from the measurement record^[3].

For Hamazaki, however, the result is not simply about fluctuations. It is another example of how seemingly different phenomena can reveal unexpected connections.

Is there a universal structure hidden beneath the complexity of these systems? For Hamazaki, a discovery is rarely the end of a question. It is often the beginning of the next one.

Exploring the Unknown, Together

Hamazaki does not describe his research as a journey toward a single final answer.

“Nonequilibrium systems are so diverse that we do not even know whether there is a universal law that can unify all of them.”

Instead, he explores different problems within the broad framework of nonequilibrium physics, looking for unexpected connections along the way.

“I tend not to focus on only one theme. I look at different problems within the broader area of nonequilibrium physics. When you do that, sometimes things that did not seem connected suddenly turn out to be related.”

Some of those connections emerge through discussion. Hamazaki’s team brings together researchers with different backgrounds, including quantum many-body physics, statistical mechanics, thermodynamics, quantum measurement, and other areas of theoretical physics.

“When we discuss things as a team, connections with other fields sometimes emerge from completely unexpected places.”

The same is true of the broader research environment at PRI.

“PRI brings together researchers from a really wide range of fields.”

For Hamazaki, one of the strengths of the institute is the opportunity to encounter ideas that might otherwise remain outside his field of view.

“You can hear about areas that you would normally know nothing about—or topics that you might never even have thought to ask about. Being able to talk with people like that is extremely valuable.”

Not every conversation immediately leads to a collaboration. But the opportunity to encounter unfamiliar ideas and speak with researchers outside his usual field is, for Hamazaki, one of the most valuable aspects of the environment.

As a child, Hamazaki enjoyed creating mathematical problems and trying to solve them. Today, he is still drawn to questions without clear answers.

“I want to keep exploring places that have not yet been explored.”

In a world full of complexity, Hamazaki continues to look for what connects it.

References:

1. K. Adachi, R. Iritani, and R. Hamazaki, “Universal constraint on nonlinear population dynamics,” Commun. Phys. 5, 129 (2022). https://doi.org/10.1038/s42005-022-00912-4
2. K. Mochizuki and R. Hamazaki, “Measurement-Induced Spectral Transition,” Phys. Rev. Lett. 134, 010410 (2025). https://doi.org/10.1103/PhysRevLett.134.010410
3. K. Yamamoto and R. Hamazaki, “Measurement-Induced Crossover of Quantum Jump Statistics in Postselection-Free Many-Body Dynamics,” Phys. Rev. Lett. (2026). https://doi.org/10.1103/wv5b-r6sb

Nonequilibrium Quantum Statistical Mechanics RIKEN Hakubi Research Team: https://sites.google.com/view/noneqquantstatmech/