In recent years, quantum simulation research has seen great success through artificial quantum systems such as ultracold atomic gases, trapped ions, and superconducting circuits. Among these, ultracold atomic gases have attracted significant attention since the first realization of Bose-Einstein condensation in 1995. In this project, we have conducted theoretical studies on quantum condensed matter physics, with results that can be experimentally verified using ultracold atomic gases.

One of the key achievements of this project, conducted in collaboration with researchers from RIKEN, the Waseda Institute for Advanced Study, the University of Tokyo, and the Chinese Academy of Sciences, is the study of magnon dynamics in ferromagnetic junctions realized with a two-component Bose gas on an optical lattice. Using the nonequilibrium Green’s function approach, we demonstrate a divergence in spin conductance and a slowdown in spin relaxation. Furthermore, quantum-enhanced conductance leads to the breakdown of the magnonic Wiedemann-Franz law. These anomalous spin dynamics arise from the magnonic critical point, where magnons become gapless due to spontaneous magnetization. The corresponding papers have been published in Physical Review Letters and Physical Review A.

Additionally, we have also investigated open quantum systems realized with ultracold atomic gases, such as quantum transport systems and ultracold atomic gases inside an optical cavity. In quantum transport systems, we have examined how atom losses introduced in ultracold atomic gas experiments affect conduction properties. In the case of ultracold atomic gases inside an optical cavity, we have studied quench and ramp dynamics near the transition point between the superfluid and density wave phases. We are currently preparing papers on these topics.