Dr. Wataru Mizukami, Associate Professor, Center for Quantum Information and Quantum Biology

Dr. Wataru Mizukami, Associate Professor, Center for Quantum Information and Quantum Biology

"The reality of quantum computer use: Computational chemist pursues quantum-classical hybrid algorithm"

Dr. Wataru Mizukami is a chemist actively engaged in the development of new algorithms for computational chemistry. He has been developing his research in quantum computing since moving to Osaka University, where the Center for Quantum Information and Quantum Biology (QIQB), the largest institute of its kind in Japan, hosts diverse research groups.

His research background is in chemistry, a field of study closely related to our daily lives which includes topics such as pharmaceutical and other industrial products and metabolism inside the living body. It is said that approximately 40% of all calculations currently performed with supercomputers pertain to chemistry, which has created a large demand for calculations for the prediction of complex chemical reactions. However, many mechanisms of chemical reactions remain unclarified even with the supercomputers currently available.

Quantum chemistry has been accumulating research results over the past 90 years since Paul Dirac, a Nobel Prize in Physics laureate in 1933, developed the principles of quantum mechanics, raising expectations for the application of quantum computers to this field.

The achievements by Dirac were one of the reasons that led Dr. Mizukami to pursue his career as a researcher. Dirac graduated from the University of Bristol in the UK, which produced multiple Nobel laureates and is renowned for its pioneering research in quantum mechanics. Dr. Mizukami spent three years there as a postdoc, starting as a visiting scholar for a year and as a prestigious Marie Curie Research Fellow in the following years. It was his privilege to experience various research environments and aspects of being a researcher, including efficient research management and good work-life balance, at an early stage of his career.

Making invisible chemical reactions visible through calculation

Dr. Mizukami says: “Some of the difficulties we face in chemistry include molecules being too small and molecules being too fast. For example, the diameter of a hydrogen atom is 1.1 ten-billionth of a meter (1.1×10-10 m). To put this into perspective, if you expanded 1m to be the diameter of the earth, the hydrogen atom would only be about the diameter of a spaghetti (1.4 mm). This is akin to viewing spaghetti on the ground from space; it is quite small. Furthermore, when hydrogen and oxygen vibrate, their movement speed is one 100-trillionth of a second (10×10-15 sec). If this period were counted as 1 second, then one second in our world would correspond to 3.2 million years. As can be seen, molecules are unfathomably small and fast-moving, making their observation immensely difficult. However, if simulation by calculation is possible, we can reproduce the chemical reactions at a scale macroscopically visible to us. This could provide us with an ‘ultimate microscope’ that converts the invisible world into one visible in detail.”

Breakthrough with a quantum-classical hybrid approach

Dr. Mizukami says: “Chemical reactions are governed by the delicate difference in energy at the molecular level. One advantage of computational chemistry lies in that such a small energy difference can be argued about in this field.” However, he adds: “The precision of chemical calculation required for analysis of such a delicate energy difference is said to be 99.99%. This is slightly higher than the precision (approximately 99.8%) of current standard quantum chemical calculation and resembles a weather forecast, which may be off at a relatively high probability. It is still at a stage where we cannot fully trust in the findings and must be suspicious when checking the results of calculations.“

Under such circumstances, Variational Quantum Eigensolver (VQE), an algorithm composed of a mixture of quantum and classical elements to make up for the shortcomings of both quantum computers and classical computers, was introduced.

Quantum computers can perform complex calculations, but are highly susceptible to noise, resulting in a short period of normal functioning. VQE works like this: (1) the results of calculation with a quantum computer are tentatively fed into a classical computer; (2) the quantum computer later utilizes the results stored in the classical computer to continue the calculation further; and (3) this sequence is repeated to manifest an effect identical to that of long-term continued operation of a quantum computer.

“No one in the 1970s or 1980s anticipated that quantum chemical calculation would be proven useful later. Chemical calculation with a quantum computer is also in its infancy. What will happen 10 years later is known to no one. Science does not advance linearly. As I am a chemist, I can feel satisfied if there is progression in chemical calculation. In those days, I thought that quantum computers should be used to cross this hurdle. However, even if a computer’s power is improved, it will be useless unless it is accompanied by progression of the algorithm. I wish to create tools useful for researchers in order to contribute to advances in chemistry on the whole.”

What is research to Dr. Mizukami?

“For me, research is like a journey. There is a preset goal, but the traveler does not know the path to reach it. Some may call this wandering, but while I walk, I can see what was invisible to me before, and my heart pounds in excitement. Sometimes I am fatigued after walking, but even so, research is a pleasure.”

Dr. Wataru Mizukami, Associate Professor, Center for Quantum Information and Quantum Biology

Explore Dr. Mizukami’s work:
Yoshioka, N. et al. (2022). Variational quantum simulation for periodic materials. Physical Review Research. 4, 013052.
Yoshioka, N. et al. (2021). Solving quasiparticle band spectra of real solids using neural-network quantum states. Communications physics. 4, 106.

Center for Quantum Information and Quantum Biology

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