Toward real application of quantum sensing and metrology

“Quantum sensing and metrology” is a relatively new research field. However, this research field is growing rapidly because of some outstanding features that pre-existing technologies do not have. We can thus achieve sensing with extremely high sensitivity across wide dynamic ranges, such as magnetic fields and temperature, or extremely accurate measurements of factors such as gravity, time, and position using quantum sensing and metrology. Multiple sensing of magnetic fields and temperature is also one of the attractive features of quantum sensing. In addition, the information with nanometer ranges extracted in local areas can be observed using nanoparticles with quantum sensors since only one spin defect can act as a sensor. These features for quantum sensing and metrology open new doors to a wide variety of fields and, as a result, ideas for new applications beyond our present imagination. Although groundbreaking demonstrations have been previously reported (Kucsko et al., 2013; Tetienne et al., 2017; Thiel et al., 2019), it is difficult to say that technology for quantum sensing andmetrology is well developed at present. The quality of host materials for spin defects that act as quantum sensors should be improved. For example, diamond is a host material for the negatively charged nitrogen-vacancy (NV) center, which is one of the most famous spin defects that acts as a quantum sensor (Balasubramanian et al., 2008). At present, there is no technology to fabricate diamond wafers of large diameters. Besides, we must develop controlling methods for reducing crystal defects, including unintentionally doped impurities, although the quality of diamond substrates improves day by day. Of course, diamond is not only a host material for spin defects but also other materials, such as silicon carbide (SiC), Gallium nitride (GaN), and hexagonal boron nitride (hBN), are expected to be applied to host materials (Ohshima et al., 2018; Gottscholl et al., 2021; Hoang, 2022), and researchers are making a significant effort to improve the quality of suchmaterials. New host materials for spin defects as well as new spin defects themselves will be found in the future and, as a result, the applications of quantum sensing will be expanded to cover a broad range of fields. In addition, it is important to establish methodologies for introducing spin defects in host materials. So far, two major methods are applied to the introduction of such spin defects during crystal growth and energetic particle irradiation (Balasubramanian et al., 2009; Yamamoto et al., 2013). Introducing spin defects during crystal growth has an advantage from the point of view of the quality of spin defects as well as host materials since unexpected residual defects that have a harmful impact on spin defects are also introduced by irradiation. For sensing with extremely high sensitivity, spin defects with OPEN ACCESS