On April 1, 2026, the Super-Kamiokande (SK) detector will mark the 30th anniversary of the start of its observations. Since its commissioning in 1996, SK has achieved numerous important results through the observation of solar, atmospheric, and accelerator neutrinos, including the discovery of neutrino oscillations. However, during this period, no core-collapse supernova has occurred within the Milky Way, and the observation of the vast number of neutrinos emitted during such an explosion has not yet been realized. Detecting neutrinos from a supernova explosion remains one of the most significant challenges in neutrino astronomy, offering insights into the explosion mechanism and the equation of state of high-density nuclear matter.

Since 2020, SK has been advancing the “SK-Gd project,” in which gadolinium (Gd) is dissolved in ultrapure water to greatly enhance the efficiency of detecting neutrons from inverse beta decay. This upgrade has dramatically improved the ability to identify supernova neutrinos and enabled the ongoing search for the first-ever detection of the diffuse supernova neutrino background (DSNB), originating from all past core-collapse supernovae in the universe. The observation of DSNB will provide valuable clues not only to the average properties of core-collapse supernovae, but also to the history of black hole formation, which cannot be probed by light. It will also allow for an independent test of key assumptions in cosmological models, such as the star formation rate and the initial mass function (IMF), from the neutrino perspective.

Moreover, in the event of a supernova explosion within our galaxy, the prompt detection of neutrinos along with directional information will be of critical importance for multi-messenger astronomy. Since neutrinos are emitted earlier than light, their early detection can serve as a trigger for follow-up observations using optical and X-ray telescopes. SK is currently the only detector capable of determining the direction of incoming neutrinos, making it an irreplaceable part of global alert networks such as NASA’s Gamma-ray Coordinates Network (GCN) and the IAU’s Transient Name Server (TNS).

This article presents the significance of neutrino detection in the study of core-collapse supernovae, highlights the latest developments with SK-Gd, and discusses future prospects for Super-Kamiokande as we enter the Hyper-Kamiokande era.

Electric multipoles are fundamental quantities in condensed matter physics. However, the precise description has not been establish even for dipoles in general systems under the periodic boundary condition, and it is more controversial for higher order multipoles. We discuss multipole insulators by introducing multipole indices based on their responses to external electromagnetic fields. The multipole index is shown to be a topological order parameter characterizing the higher-order topological insulators.

The highest record of superconducting transition temperature at ambient pressure was set by the Hg-based cuprate more than 30 years ago, but its electronic structure relevant to superconductivity has remained largely unsettled. Overcoming experimental difficulties, angle-resolved photoemission spectroscopy has recently been applied for the first time to the Hg-based cuprate. In this review, the unveiled electronic structure in the superconducting state is compared with the case of other cuprates, and the key to its high transition temperature is discussed.

A superconductor-insulator transition (SIT), which is induced by a magnetic field in disordered thin films, is a famous example of a quantum phase transition (QPT). However, in some weakly disordered or crystalline thin films, an anomalous metallic (AM) state appears between the superconducting and insulating phases. The origin of the AM state and how its emergence modifies the usual QPT picture remain unresolved. To examine these issues through quantum fluctuations, we measure the Nernst effect, which is highly sensitive to fluctuations of the superconducting order parameter. We demonstrate the existence of a quantum flux liquid within the AM state and identify a quantum critical point inside it, indicating that the AM state is a broadened critical state of the SIT.