Towards detecting single spins, imaging spins and sensing biomaterial using a single superconducting artificial atom

After developing a sensitive electron spin resonance spectrometer, this research field has expanded in many directions. The most straightforward one was sensitivity improvement. We have been adding new functionary in spin imaging. We have expanded the target material to include biomaterials.

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 Publishing our paper on spin sensing using a superconducting artificial atom [Toida et al., Communications Physics, 2, 33 (2019)] has accelerated both my scientific research and also career. It has enabled collaboration with many world-leading experts.

 After the publication of the paper, I got many opportunities to publicize our research through an invited talk at an international workshop on quantum science and a prestigious award within our company. These were a good chance to expose our work to researchers outside of the superconducting qubit community and are notable achievements in my research career.

 Surprisingly, a rare occasion occurred where I could introduce our work to a national audience. This occurred by chance with our work being shown in a TV program. It was a non-scientific program, mainly focusing on my life and achievement.  Fortunately, I was given the opportunity to explain our spin sensing project and also show our laboratory and its facilities. After broadcasting, I got many messages from some of the audience through social networking services. Those were the real voices of expectations for scientists and engineers from the outside of the scientific community and it was a chance to reconfirm the connection between scientific work and society.

 Our spin sensing project is ongoing and involves collaborations with many experts. This will give many more opportunities to present our spin sensing project and its results.

 We have recently achieved one-order magnitude improvement of the sensitivity enabling us to detect as few as 20 spins as we report in early 2020 [Budoyo et al., Applied Physics Letters, 116, 194001 (2020), Featured Article]. We were excited to reach this nice number, however, we have not reached our ultimate goal of 1 spin. We are still not utilizing the full power of the qubit: we have room to improve the sensitivity down to our goal by using the quantum coherence (superposition states) of the flux qubit.

 The figures of merit like sensitivity are important in this metrology field, however, the functionalities of the sensor should also be considered for specific applications. This could include determining the distribution of spins. We can imagine magnetic resonance imaging (MRI) for medical diagnostic applications using this approach. For this spin imaging, the design of the basic 16 x 16 qubit array device has been completed, however, there are other difficulties associated with the wiring of so many qubits as our current dilution refrigerator is not setup to handles such a large number of wires. Thus, signal multiplexing is an important task required in implementing this qubit array system. We have just started a collaboration with an expert on superconducting logic circuits to design a multiplexer using superconducting devices at low temperatures.

 Another challenging project is associated with our desire to perform spin sensing in biomaterials. The project is an ongoing collaboration with the bioscience group within our laboratories as well as experts in the field of electron spin resonance (ESR) from a Japanese university. We were facing difficulties in combining two extremely different materials: hard and soft ones. Direct attachment of the biomaterial to the flux qubit kills the superconducting device completely. After trials of several types of insulators between them, we have finally been able to measure the magnetization from spins in cells as we reported in the American Physics Society March Meeting 2020. We are now in the process of measuring the ESR spectrum to identify the spin species.

 Our spin sensing paper has been one of the milestones of our project. It has spawned a bunch of very interesting and important related works. Our paper has led to many collaborations with international experts. It has made my research life much more exciting.

Hiraku Toida

Research Scientist, NTT Basic Research Laboratories

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