Exploring the magnetic properties of various materials is a central topic in condensed matter physics. In recent years, stimulated by spintronics and atomically thin materials research, the importance of methods to investigate magnetic properties in micro- or nano-sized magnetic materials has increased. Various techniques have been developed to measure local magnetic fields or magnetization, including Lorentz electron microscopy, magneto-optical Kerr microscopy, magnetic force microscopy, scanning Hall probe microscopy, and scanning SQUID microscopy. Such various methods speak to the fact that a definitive method does not yet exist. Thus, local magnetic measurements offer an attractive challenge to experimentalists. 

Recently, there has been considerable interest in using nitrogen-vacancy centers (NV centers) in diamonds as quantum magnetic sensors. Since NV centers have a spin-dependent optical process, their spin state can be read out optically by combining lasers and microwaves, so-called optically detected magnetic resonance (ODMR). As the ODMR spectrum of NV centers in a magnetic field shows a Zeeman splitting, the magnetic field they feel can be deduced by analyzing the spectrum. A similar ODMR in boron vacancy defects in hexagonal boron nitride (hBN) was reported in 2020.

We aim to use the diamond and hBN quantum sensors to measure the magnetic properties of various materials locally and quantitatively. The interest of this quantum sensor research is that it is possible to study from both viewpoints of “quantum control” and “quantum sensing.” 

In this talk, we will introduce our recent works on quantum sensors, such as 

(1) Geometric diabatic control of quantum states [1]

(2) Observation of superconducting quantum vortices [2]

(3) Chiral magnetic domain wall in antiferromagnet Mn3Sn [3]

(4) Quantum sensing enhanced by machine learning [4]

(5) hBN quantum sensor nanoarrays [5].

We hope this talk will provide an opportunity to convey the hopeful future of quantum sensors in condensed matter physics.

 

[1] K. Sasaki et al. Phys. Rev. A 107, 053113 (2023). 

[2] S. Nishimura et al., Appl. Phys. Lett. 123, 112603 (2023). 

[3] M. Tsukamoto et al. (submitted). 

[4] M. Tsukamoto et al., Sci. Rep. 12, 13942 (2022). 

[5] K. Sasaki et al., Appl. Phys. Lett. 122, 244003 (2023).