This story started in early 2013, when I joined the superconducting circuit research group in NTT just after finishing my Ph.D. The focus of the research of the group I joined at that time was hybridizing superconducting flux qubits with electron spin ensembles to realize quantum memories for those flux qubits (quantum processor). Actually, coherent transfer of quantum information between the flux qubit and NV centres in diamond had been successfully observed in the group [Nature 478, 221–224 (2011)]. As the first project in my professional research career, I was exploring the hybridization of a flux qubit with erbium spins in a Y2SiO5 crystal. However, after one and half years of trial, I could not observe significant evidence of such hybridization. Taking this failure into account, we began discussing future projects. During the discussion, we noticed that the experimental setup used for the hybridization of a flux qubit and a spin ensemble would be an ideal apparatus to realize a brand-new electron spin resonance (ESR) spectrometer.
ESR spectrometers are commonly used to analyse the properties of electron spins in many research fields including physics, chemistry, biology and geoscience. (Actually, I have used it once for my undergraduate chemistry experiment course to analyse anthracene molecules.) A microwave cavity is a detector of a signal from electron spins for standard ESR spectrometer. However, here, we instead use the much more sensitive superconducting flux qubit as a detector of spins (magnetometer) unlike the standard ESR spectrometer. This kind of usage of a qubit in this case is a little bit different from its standard usage in a quantum information processor. The flux qubit shows excellent characteristics as a magnetometer: high sensitivity and high spatial resolution. It responses to a magnetic flux of mΦ0 (Φ0 is the flux quanta), while the conventional superconducting magnetometer, a superconducting quantum interference device (SQUID), responses to magnetic flux of Φ0. This leads to high sensitivity in our case. The size of the flux qubit can be shrunk down to micrometers or even to the nanometer scale. This leads to both a high sensitivity and a small detection volume. After detailed analysis of the experimental results of our ESR spectroscopy, the sensitivity and the spatial resolution of our ESR spectrometer found to be comparable to that using state-of-the-art superconducting technologies. At that time, the best sensitivity and detection volume was reported to be 150 spins/√Hz and 20 pL by CEA Saclay group [Nature Nanotechnol. 11, 253–257 (2016)]. Considering the sensitivity and the sensing volume have trade-off relationships, our values of 400 spins/√Hz and 0.05 pL showed clear advantages.
ESR spectrometer with high sensitivity and spatial resolution will open up new research avenues. To expand potential applications of our ESR spectrometer, we are currently working on the next generation projects: detecting a single spin, characterizing transition metal ions in biomaterials, and developing “spin camera” like CMOS image sensor.