What is the gravity of a quantum system and what does it cause? This is a fundamental problem for unifying quantum mechanics and general relativity, which has not been elucidated yet. A key feature for elucidating the problem is the quantum superposition of a gravitational field, which may appear in the superposition of a quantum system with a mass. Interestingly, this quantum phenomenon of gravity generates entanglement in quantum systems. This gravity-induced entanglement can be a main target in the above context. In this article, we share the current studies on the gravity of a quantum system and on gravity-induced entanglement.

Ruthenium compounds have served as a playground for fundamental concepts in modern condensed matter physics, encompassing unconventional superconductivity, Kitaev spin liquids, and solid-state analogues of the Higgs mode in particle physics. Quantitative description of these exotic phenomena requires experimental information on the interaction strengths among the 4d electrons. Our development of a new resonant inelastic X-ray scattering (RIXS) instrument dedicated to the tender X-ray regime (2–5 keV) allows systematic investigation of magnetic excitations in 4d transition metal compounds. The single magnon dispersion of a high-temperature antiferromagnet SrRu₂O₆ demonstrates the critical role of spin–orbit coupling in stabilizing the antiferromagnetic order. The magnetic excitation spectra of a Kitaev spin liquid candidate α-RuCl₃ points to the dominance of bond-dependent Kitaev interactions. Our results exemplify the capability of state-of-the-art RIXS experiments combined with many-body theory calculations in providing insight into exotic magnetism in prominent model compounds.

Fat bloom, which is the whitening of chocolate, is a serious problem in confectionery. The common reason for the blooming is the polymorphic transition of cocoa butter crystals. We found a novel mechanism for the fat bloom of chocolate without polymorphic transition. When chocolate was heated to 35–37℃, the melting point of cocoa butter crystals became higher, or the cocoa butter crystals transformed into the most stable polymorph. Cocoa butter was recrystallized with remaining seed crystals to form desirable polymorph after cooling to 20℃. Depletion of cocoa butter from the surface of chocolate caused by recrystallization promoted void formation and blooming.

Recently, temperature observations of neutron stars have gained attention as a means of exploring physics beyond the Standard Model of particle physics. The cooling of isolated neutron stars, through neutrino and electromagnetic radiation, is understood as the standard cooling theory, which generally aligns with observational data. However, the presence of hypothetical particles such as axions and dark matter, predicted by theories that extend the Standard Model, could alter this cooling behavior. Axions, for example, increase cooling rates, while dark matter interactions could lead to additional heating. By comparing revised theoretical predictions with observed temperature evolution, scientists hope to detect signs of these elusive particles.