Focus Issue on Quantum Matter

Research on the quantum nature of matter and light has flourished in recent years, as the field has grown for theorists and experimentalists alike. New research directions, such as the study of novel phases in quantum materials, have laid the ground for new foundational theories and future quantum technologies. In particular, novel topological materials and superconducting hybrid systems are promising for the development of novel superconductors and quantum computers. This Focus Issue aims to highlight some of the latest achievements in the field of quantum matter. It includes 3 Reviews, 1 Perspective, and 3 Research Articles on a range of topics, including single-photon emitters for quantum technologies, topological quantum materials, and quantum sensing and metrology: Rare-Earth Doped Thin Films for Optical Quantum Technologies (Philippe Goldner et al., 10.1002/qute.202500026): A Review article on rare earth-doped thin films—materials that are highly promising for use in on-chip photonic circuits thanks to their excellent coherence properties. The integration of rare-earth thin films into photonic platforms could potentially enable photonic quantum technologies, such as optical quantum memories, which are relevant for communications and processing. Cuprate twistronics for quantum hardware (Nicola Poccia et al., 10.1002/qute.202500203): A topical Review on the emerging field of cuprate twistronics, which involves stacking and twisting ultrathin layers of cuprate superconductors. This technique creates Moiré patterns and new electronic states, uncovering new opportunities in both applied (e.g., quantum hardware) and fundamental physics. Quantum spin Hall effects in van der Waals materials (Qiong Ma et al., 10.1002/qute.202500327): A Review of the Quantum Spin Hall (QSH) effect in two-dimensional van der Waals (vdW) materials. The QSH effect is promising for the development of low-power electronics and topological quantum computing. The article explores the main developments in the field, emerging research directions, as well as experimental challenges. Beyond kagome: p-bands in kagome metals (Alexander Tsirlin et al., 10.1002/qute.202500336): A Perspective arguing that p-bands (electron bands from non-transition metals) play a critical, often overlooked, role in the electronic instabilities, such as charge-density waves and superconductivity, observed in kagome lattice metals. High-purity single-photon emission in the telecom O-band from droplet-epitaxy InAs quantum dots integrated into a GaAs/AlGaAs planar microcavity on vicinal GaAs(111) (Battulga Munkhbat et al., 10.1002/qute.202500159): A Research article on developing a reliable source of single photons in the telecom O-band (a critical wavelength for fibre communication) using InAs quantum dots grown via droplet epitaxy. Single-photon emission from such sources could potentially be useful for building global quantum communication networks. Can Janus monolayer transition-metal dihalides be bright excitonic insulators? (Yuanchang Li et al., 10.1002/qute.202500345): This Research article explores Janus monolayers (asymmetric 2D materials) as potential candidates for bright excitonic insulators (EIs). EIs are a state of matter where electrons and holes spontaneously form excitons that condense, and bright EIs could provide a unique platform for coherent quantum optical devices. Highly sensitive magnetometer with continuous excitation Ramsey protocol by utilizing long dephasing time of millimetre-scale spin ensembles in (111) diamond (Mutsuko Hatano et al., 10.1002/qute.202500056): This Research article presents a method to significantly enhance the sensitivity of magnetometers by utilizing the long dephasing time of nitrogen-vacancy (NV) centre spin ensembles in a specific orientation of diamond. NV-diamond magnetometers are key for high-precision quantum sensing. Through this Focus Issue, we aim to highlight the major advances in quantum matter research and to offer perspectives on key directions for future research. We hope that readers will find the content of this collection both inspiring and useful.