A brief review on the fundamental properties of quantum entropies, as well as their short histroy, is presented. The main focus is the connection between information and thermodynamics on the basis of the von Neumann entropy and the quantum relative entropy.

Quantum field theory describes the dynamics of quantum many-body systems by including the creation and annihilation of particles, using “fields” spread over the entire space-time as fundamental variables. It has been established as a fundamental theory of high-energy physics and condensed matter physics, producing numerous achievements. Recently, ideas of quantum information have been employed to understand quantum field theory geometrically. In this article, I will explain such developments by focusing on the monotonicity of degrees of freedoms, such as the entropic c and g-theorem.

Primordial gravitational waves, predicted by quantum theories of spacetime, are a key probe of the beginning of the universe. Their detection is expected through precise measurements of the cosmic microwave background (CMB) polarization, though the signal is extremely weak. To observe it, a large format of superconducting Transition Edge Sensors (TES) are deployed. This paper outlines the early universe revealed by CMB studies, the development of large-scale TES focal plane arrays, and recent efforts by the newly established WPI Center for Quantum Field Measurement Systems (QUP) to advance spacetime observation using quantum sensors.

In the field of surface science, techniques for creating and observing atomically clean solid surfaces have been well established. Furthermore, recent advancements in measurement methods based on scanning probe microscopy have made it possible to manipulate individual atoms and molecules on solid surfaces and directly investigate quantum phenomena arising in these artificial systems. In other words, it is no longer a distant dream to explore quantum physics and create quantum functionalities in solid-state systems where both the type and relative positions of all constituent atoms forming the functional core of the system are precisely defined. This article introduces and explains recent research trends while providing an outlook on future developments in this field.

Topological quantum materials, those that are characterized by their topological properties, have been a central theme in quantum condensed matter physics. Canonical examples include quantum Hall systems, topological insulators and superconductors, and the Haldane phase in quantum spin systems. Recently, the concept of topology has been playing a pivotal role in studying quantum many-body systems. In this article, we review some of the key developments and concepts in topological condensed matter systems.

Quantum phases in solids―superconducting phase, for example―suddenly appear and fascinate us. These changes are driven by phase transitions, which are induced not only by thermal fluctuations but also by quantum fluctuations. And under conditions with strong quantum effects, they are difficult to predict and calculate. Understanding the known patterns of quantum phase transitions induced when solids are controlled by external parameters will lead to the discovery of novel quantum phenomena. After a simple review of phase transition phenomena, this article gives examples of complex and fascinating quantum phase transitions induced by electrons in solids.

Recent progress in ultracold atomic physics provides us with an interdisciplinary way to address neutron-star matter from table-top experiments. Based on the many-body properties in Fermi atomic polarons, we discuss possible ³P₀ neutron superfluid in dilute neutron matter and polaronic protons in asymmetric nuclear matter.

Charged lepton flavor violation (CLFV) is a key to understanding new physics beyond the Standard Model. The muonium-to-antimuonium (Mu-to-Mu¯ ) transition is one of the CLFV processes, which is sensitive to new physics that changes both electron and muon flavors by two units. The Mu-to-Mu¯ transition can be enhanced in models with CLFV interaction involving doubly-charged or neutral bosons, and the relation to neutrino mass models is interesting. This article focuses on the potential for discovering new physics in next-generation experiments of the Mu-to-Mu¯ transition.