Stochastic computing, which actively uses fluctuations of physical quantities, has attracted much attention. The relaxation speed of the fluctuations determines the computational speed and accuracy. Focusing on the thermal fluctuations of the nanomagnet, we theoretically investigate the relaxation time of superparamagnetic tunnel junctions (s-MTJs), which serve as “probabilistic bit” in spintronics-based stochastic computing. We find that the attempt time, which is usually assumed to be a constant in the Néel-Arrhenius law describing the relaxation time of magnetic materials, significantly varies depending on the s-MTJ device structure. We build a guideline for a fast relaxation time and demonstrate the s-MTJ device with a relaxation time of 8 ns, 100 times shorter than the shortest relaxation time ever achieved.

The simplicity of black holes is guaranteed by the no-hair theorem in general relativity. Binary black hole mergers that lead to the emission of gravitational wave (GW) ringdown are important to test general relativity in the extreme gravity regime. The recent data analysis of GW ringdown revealed that we could extract the multiple quasi-normal modes of a black hole from GW ringdown with high accuracy if the emission of GW ringdown starts earlier. Also, it turned out that there is a universal feature of the excitation of multiple quasi-normal modes, which was confirmed by both the data analysis of simulated GW waveforms and the black hole perturbation theory. The emission of GW echo could be caused by quantum effects near black hole horizons and can be tested by the third-generation GW observatories.

The normal- and superconducting-state properties in the recently discovered U-based superconductor UTe₂ were investigated with the nuclear-magnetic-resonance (NMR) measurements. The unusual NMR spectrum broadening observed at low temperatures suggests the presence of slow dynamics, which is seemingly a precursor of some ordered state. From the Knight-shift measurements in the superconducting state up to 24 T, the possible superconducting states in the low-field and high-field are suggested.

We have found that two amorphous solid states can exist in ⁴He obeying the Boltzmann statistics under compressed conditions. The states are distinguished by quantum wavelength of atoms, and are called low and high quantum dispersion amorphous solids (LQDA and HQDA). The existence of the two states indicates the quantum polyamorphism in this system. The state transition occurs reversibly by varying pressure or temperature, while it resembles the coil-globule transition in classical polymer systems. The atomic diffusion in HQDA is enhanced by compression or cooling. This enhancement of atomic diffusivity is explained by the existence of atomic tunneling in the HQDA. The present finding suggests that this new type of polyamorphism is involved in the real ⁴He.