Quantum Field Theory R&D – Analysis and Prediction of Strongly Correlated Quantum Many-Body Systems

As one of the projects to commemorate the 75th anniversary of the founding of Keio University's Faculty of Science and Technology, the University has launched the Keio Institute of Pure and Applied Sciences, commonly referred to as KiPAS. At KiPAS, groundbreaking efforts are being made by providing four chief researchers who are focused on fundamental academic research fields with environments that facilitate commitment to research. Those researchers were selected from full-time faculty members of the Faculty of Science and Technology.

One chief researcher at KiPAS is Professor Yoji Ohashi, in the Faculty of Science and Technology. Together with Dr. Daisuke Inotani, Professor Ohashi is doing theoretical research on quantum condensation phenomena, primarily superconductivity and superfluid phenomena, where quantum interactions between particles play an important role.

"My research theme is the construction of a theory that enables quantitative research on strongly correlated quantum multi-body systems. In modern physics, the quantum nature of particles and interactions between particles are extremely important. For example, the phenomenon of superconductivity occurs because interactions between particles create molecules called Cooper pairs, which results in a quantum condensate called a Bose-Einstein condensate. In that case, interactions between particles are a very important point; it’s thought that increasing the strength of the interaction that creates Cooper pairs is the key to achieving room-temperature superconductivity, which people haven’t managed to do yet."

Currently, when an electric current is passed through a metal at room temperature, electrical resistance occurs, so when electricity is transmitted, a large amount is lost. But if the metal is cooled to an ultralow temperature, below minus 100 degrees C, so it enters a superconducting state, the electrical resistance disappears completely. This makes it possible to transport electricity efficiently, without power loss. So, if the superconducting state can be achieved at room temperature, revolutionary advances can be expected, not only in fundamental science, but also in technology.

Professor Ohashi considers that, to achieve humanity’s dream of room-temperature superconductivity, we need a theory that adds quantum aspects to interactions between particles, which are difficult to handle theoretically. So, he’s working to construct a reliable theory of strongly correlated quantum many-body systems, by utilizing analytical and numerical computation.

"We do some calculations by hand, but with that kind of analytical computation alone, it’s hard to bring in interactions at the level where the theory can be compared with reality. So, for calculations that can’t be done by hand, we use numerical computation as well as analytical approach. Our approach is to bring the computation to a level where it can be compared with experiment, by using a computer."

Professor Ohashi is also extending his research to the superfluidity that occurs in a ultracold atomic Fermi gas, an artificial quantum many-body system that has been achieved recently.

An ultracold atomic Fermi gas has the revolutionary advantage of enabling free, precise experimental control of various material parameters that dominate the system’s properties, notably the interaction between particles, which plays a crucial role in achieving a superfluid state. By utilizing this feature, researchers can verify in detail whether a theory is appropriate, and what its problems are, using an ultracold Fermi gas – a quantum multi-body system that actually exists.

"Among ultracold atomic systems, I’m researching superfluid phenomena, using p-wave interactions. In particular, I’m incorporating the fact that theoretically, when such interactions become strong, an effect called pairing fluctuation effect becomes important. I’m using a computer to calculate various physical quantities, for example, the density of states, spectral intensity, and phase diagram."

"For example, experimentally, a p-wave superfluid hasn’t been achieved, but it’s possible that we can find out theoretically how far we need to decrease the temperature to achieve such a superfluid phase, and if we achieve it, what properties the system has in the superfluid state."

"Currently, progress is being made in various experiments with cold atomic Fermi gases, and various thermodynamic measurements have become possible. It’s become clear that we can’t solve problems just by incorporating interaction effects into our theories. How can we overcome this issue? Right now, we have to think about that from the theoretical fundamentals, but unless we overcome this issue, the theory of superfluidity in fermion systems ultimately won’t be complete. So, I’d like to find a way to overcome it, somehow, within the next five years."