Many-body theory of light and matter

Our main research interest is in the intersection between condensed matter physics and AMO (atomic, molecular, and optical) physics.

With recent progress in the technology of AMO physics, people can now realize various phenomena, models, and Hamiltonians using atoms and molecules controlled by light and/or light itself. Such systems are sometimes called synthetic quantum matter or quantum engineered systems. Our primary interest is to understand many-body theory in AMO-related platforms. Such platforms include ultracold atomic gases, photonic crystals/lattices/cavities, polaritons, etc. We explain some of our previous research below.

Topological physics of light and matter

One of our focus now is to understand analogs of topological insulator physics in light and matter. The term "insulator" is most suited for charged fermions such as electrons, but bosons can take a (single-particle) energy spectrum which is similar to that of topological insulators. Then, features characteristic of topological insulators, such as bulk-edge correspondence, can also be observed in bosonic systems. Study of topological physics in photonic systems is called topological photonics. Photonic systems can show characteristic topological phenomena not available in electronic systems, such as the interplay between the topology and dissipation/gain, and lasing from topological edge states known as topological laser. For more details of topological photonics, please check our review:

As examples of more specific topics, we have studied synthetic dimensions and effects of quantum metric in synthetic quantum matter. Synthetic dimensions provide a way to simulate effects of spatial dimensions by means of non-spatial degrees of freedom, such as internal states of atoms and/or photons. With synthetic dimensions, one may even be able to simulate physical phenomena which can occur only in systems with four (or larger) spatial dimensions. Please check out our review for more details:

Quantum metric is a geometrical property of quantum states in a parameter space. Effects of geometrical properties such as the Berry phase and the Berry curvature have been studied well in the context of topological physics. Quantum metric, on the other hand, is slightly different; quantum metric provides a Riemannian metric for the parameter space induced from the quantum states, and is known to be related to localization properties of the system. Recently, quantum metric has been found to appear in various physical phenomena. We have also found effects in ultracold gases and photonics which directly comes from the quantum metric, and we have also proposed how to measure the quantum metric. Together with experimentalists, we have also been involved in the experimental measurements of quantum metric in ultracold quantum gases and diamond NV centers.

Classical simulation of quantum mechanics

Quantum mechanical particles behave like "waves" in some sense, hence the name "wavefunction." Interestingly, many of the single-particle properties of quantum mechanical systems can be simulated/reproduced using classical waves. We are interested in pursuing this analogy between the quantum mechanical waves and classical waves. Genuinely quantum concepts such as entanglement cannot be classically simulated (in an efficient way), but some phenomena known in quantum mechanics have counterparts in classical systems; a good example is the quantum Hall effect. We look for quantum-mechanics-inspired exotic phenomena in classical systems. In this direction, together with groups in RIKEN and Keio, we recently explored analogy of topological edge states in the collective motion of neural progenitor cells, which can be modeled as active nematic matter with a certain chirality.