Recent development in the atomic physics enabled us to perform extremely high precision spectroscopy, as high as 10^-18 relative precision, and with this high precision, eV-scale phenomena have an access to high energy physics. Among atoms that are commonly used for these high precision spectroscopies, ytterbium has various features that are suitable for applications for the fundamental physics: several narrowlinewidth transitions for both neutral atoms and ions, seven stable isotopes, and transitions involving f orbitals. Specifically, ytterbium has so far been used for ultralight dark matter searches by atomic clock comparisons, new boson searches through precision isotope shift measurements, and searches for time variations of fundamental constants. Further improvements in the precision of the spectroscopies are expected to enhance the regions of these searches, and features for ytterbium atoms can be utilized for investigating other new physics.

The sign problem has been a major obstacle to first-principles calculations of a variety of important physical systems. Typical examples are finite-density Quantum Chromodynamics, some statistical systems such as strongly correlated electron systems and frustrated spin systems, and the real-time dynamics of quantum many-body systems.

There have been proposed various approaches to the problem, among which the Lefschetz thimble method uses a continuous deformation of the integral surface into the complex space to tame the oscillatory behavior of integrals. The “tempered Lefschetz thimble method” (TLT method) implements the tempering algorithm with respect to the deformation parameter, and is the first algorithm that simultaneously solves the sign and ergodicity problems, which have been existing intrinsically in the Lefschetz thimble method.

The shortcomings of the TLT method regarding high numerical cost are resolved in the “Worldvolume Hybrid Monte Carlo method” (WV-HMC method), where Hybrid Monte Carlo updates are performed on a continuous accumulation of deformed surfaces (worldvolume). The WV-HMC algorithm may be a promising method towards solving the sign problem due to its versatility and reliability.

A quantum biosensor by fluorescence nanodiamond has one of potentially useful killer applications in quantum information technology. Our studies pointed out the first application of a quantum biosensor toward understanding the physiological quetsions on Caenorhabditis elegans, especially in thermal biology.

To clarify the order parameter of the non-magnetic phase transition in iridium oxide Ca₅Ir₃O₁₂ at 105 K, various experiments such as Raman scattering, inelastic X-ray scattering, and synchrotron radiation X-ray diffraction were conducted. The results showed that the order parameter is an electric toroidal dipole order.