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Professor Keisuke Fujii, Graduate School of Engineering Science

Professor Keisuke Fujii
Division of Advanced Electronics and Optical Science
Graduate School of Engineering Science

"Quantum computing – future in the making"

Despite the rapid development of digital computers, certain types of calculations, such as factoring large numbers and simulating complex molecules and chemical reactions, are difficult to perform even by the fastest digital supercomputers. Quantum computers have been proven able to perform such calculations much faster than digital computers [1]. The basic unit of a quantum computer is known as “qubit,” whose value can be 1, 0, or 1 and 0 simultaneously due to the principle of quantum superposition. Due to this property, a qubit can represent a complex state in which 0 and 1 are superimposed with various weights. Since the superposition pattern becomes exponentially more complex by increasing the number of qubits, a quantum computer with a sufficient number of qubits can outperform even the fastest digital supercomputer in certain types of calculations.


Pioneering spirit and perseverance paid off

Dr. Keisuke Fujii started his research on quantum computing algorithms during his undergraduate studies at Kyoto University. At the time, quantum computing was not yet a “hot” topic, and he could not find a laboratory specializing in it. He then decided to conduct his undergraduate research under of a high-energy particle physics professor who allowed him to research anything related to quantum physics, purchased a textbook, and taught himself the theory of quantum computing. 

Throughout his scientific career, Dr. Fujii has maintained a strong pioneering spirit. Known as one of the foremost researchers in quantum computing, he has been involved in a broad range of research projects on, as he puts it, “anything in quantum computing other than hardware.” In one of his groundbreaking papers published in 2018 [2], Dr. Fujii described the Quantum Circuit Learning (QCL) algorithm that he developed with a research group from Osaka University (he was working at Kyoto University at the time) using numerical simulations. IBM then demonstrated the algorithm on their quantum computer and published a paper in Nature a few months later [3], and the QCL paper has now been cited by more than 250 publications. He encourages young scientists who are working on pioneering theoretical subjects that may not be popular at the moment by saying that with the rapid progress of technology, it will not be long before such subjects can be implemented.


Collaboration generates new ideas and drives innovation

Dr. Fujii is currently serving as Deputy Director of the Center for Quantum Information and Quantum Biology at Osaka University, where experts from various disciplines work together in the development and applications of quantum computing. He enjoys the Center's interdisciplinary atmosphere, which he claims has broadened his horizons to pressing issues in information technology, chemistry, and biology that require quantum computing to solve. Dr. Fujii is particularly interested in developing quantum computing algorithms for machine learning and improving the efficiencies of chemical processes, such as the Haber-Bosch process. Despite the challenges in building large-scale quantum computers, he believes that they can be realized in the relatively near future and will have significant contributions to the betterment of humanity.

[1] Arute, F., Arya, K., Babbush, R. et al. (2019) Quantum supremacy using a programmable superconducting processor. Nature 574, 505–510.

[2] Mitarai, K., Negoro, M., Kitagawa, M., Fujii, K. (2018) Quantum circuit learning. Physical Review A, 98, 032309.

[3] Havlíček, V., Córcoles, A.D., Temme, K. et al. (2019) Supervised learning with quantum-enhanced feature spaces. Nature 567, 209–212.

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