The asymptotic phase is a fundamental quantity in the theory of classical nonlinear oscillators, but it has not been defined for quantum nonlinear oscillators. We have recently proposed a fully quantum-mechanical definition of the asymptotic phase for quantum nonlinear oscillators, which naturally extends the definition of the asymptotic phase for classical nonlinear oscillators from the Koopman-operator viewpoint. In this article, after briefly reviewing the analysis of classical nonlinear oscillators using the asymptotic phase and overviewing recent developments in the theories of quantum nonlinear oscillators and synchronization, we explain our recent studies on the semiclassical phase model and definition of the quantum asymptotic phase and its use for analyzing quantum synchronization in the strong quantum regime.

Multipole moments are introduced through spherical expansions of charge, spin, and current distributions in real space. However, electric toroidal multipole moments do not arise from the expansion of these physical quantities. Upon a reconsideration of microscopic quantities, it becomes clear that it is necessary to consider electric polarization arising from spin degrees of freedom and chirality operators originating from the Dirac field.

Dark matter is one of the crucial topics in both particle physics and cosmology. The mass of dark matter and the characteristics of its interactions remain unknown. The DOSUE-RR (Dark-photon dark-matter Observing System for Un-Explored Radio-Range) experiment is designed to search for dark matter, with a special focus on a specific undiscovered particle, dark photon. The dark photon has a slight interaction with ordinary photons, whose coupling constant is χ. Under the assumption that the dark photon is dark matter, the dark photon is converted into ordinary photons on a metal surface. Its direction is nearly perpendicular to the surface, and the frequency ν₀ corresponds to the mass of dark photon m_DP as hν₀=m_DP c². Our target mass range is from 40 μeV to 1 meV, which corresponds to the millimeter-wave range from 10 GHz to 240 GHz. Using a simple setup with millimeter-wave receivers under a metal plate, we successfully searched for the dark photon in the mass range from 41 μeV to 110 μeV. Our measurements showed no significant signal. However, the derived limits on χ are more stringent than those obtained from previous studies.

We report bond-breaking dynamics of a tetrahedral molecule, CH₄, in ω–2ω intense laser fields. Spatially asymmetric fragmentation is identified for H⁺ and H₂⁺ fragments with characteristic dependences on the relative phase of the two-color laser fields. The asymmetry is discussed in terms of the orientation-selective ionization and strong interstate coupling in ω–2ω intense laser fields.