Silicon Could Make Quantum Leap Possible: 30 Years of Isotope Research and Innovation

Kohei Itoh

Professor, Department of Applied Physics and Physico-Informatics, Faculty of Science and Technology

July 31, 2017

Every day we live and work on what some scientists charmingly call "classical computers"—digital machines that use bits to flip between 0s and 1s. But that may change with the advent of quantum computing. With silicon poised to become the major material of the quantum revolution, Faculty of Science and Technology Professor Kohei Itoh is uncovering the element's unique properties here at Keio. We sat down with him to hear how his world-shaking vision came about and to find out where he plans to go from here.

When you hear the word semiconductor, you probably think of the electronic components inside devices like personal computers and mobile phones. But the term semiconductor actually refers to a substance that has a conductivity between that of an insulator, which does not transmit electricity, and a conductor, which does.

There are various kinds of semiconductors in the world, but most are made of a specific element, such as germanium or silicon. And while most semiconductor research focuses on those elements, the Itoh Research Group at Yagami Campus has been garnering worldwide attention for doing things differently. Rather than focusing their research on semiconductive elements themselves, they are focusing on the isotopes within a single element, which are classified according to the difference in their neutron number.

What Are Isotopes?

Our world is made up of around 90 elements such as hydrogen, helium, and lithium. Look closely at any one element and you will find a nucleus made up of protons and neutrons, surrounded by electrons. A hydrogen atom contains one proton while a helium atom contains two, and so on. The number of protons in the nucleus is what differentiates elements from one another. But it is also understood that within a single element there can be a different number of neutrons as well. For any one element, of course, the number of protons is always the same, but with more neutrons the element will be heavier. Each of these forms of a single element—which contain the same number of protons but a different number of neutrons—is called an isotope. Isotopes can be found in the semiconductors we discussed earlier. Germanium (Ge), for example, has five naturally occurring isotopes with mass numbers (number of protons + number of neutrons) of 70, 72, 73, 74, and 76. Silicon, on the other hand, has just three stable isotopes, numbered 28, 29, and 30.

Prof. Itoh's career began with research on germanium (Ge) isotopes. In his graduate school laboratory, he studied the physical properties of the semiconductor element, which were needed to develop thermometers (bolometers) for astrophysics observation.

Itoh's research into separating isotopes required the same technology used to enrich uranium for nuclear weapons at a time when the Cold War was coming to a close. Amid concern over the protection of nuclear technologies, Western powers established research funding schemes where researchers could continue to work without fear of leaking classified Soviet information from nuclear military facilities. As a result, Itoh was able to obtain isotopically separated germanium, which is safe and nonradioactive. This allowed him to go beyond the elemental level to study the differences in physical properties at the isotopic level.

“We were the ones who came up with the phrase ‘semiconductor isotope engineering’,” Prof. Itoh says. In 1998, his lab received a grant to acquire silicon isotopes and began experimenting with growing silicon crystals. “The heart of the classical computer is its silicon semiconductor. And since it is more widely used and more versatile than germanium, we decided to start studying its isotopes. We started with making clean silicon crystals and wrote papers on the differences between all manner of physical properties, including heat flow, magnetic properties, and electrical properties.” His lab's deep, wide-ranging knowledge of silicon and its isotopes has become a springboard for subsequent applications.

While working on silicon isotopes in 1998, Prof. Itoh saw a paper that took him by surprise. The paper was B.E. Kane’s A silicon-based nuclear spin quantum computer, which detailed a way to build quantum computers using silicon atoms. Prof. Itoh immediately invited Dr. Kane to Keio to discuss in person the potential of silicon isotopes, and decided to pursue research into the application of silicon isotopes for quantum computing.

To better understand the importance of quantum computers, it helps to understand Moore’s law and the limits of the current classical computer. Moore's Law is a prediction of the growth rate of classical computers made by Gordon Moore in 1965, and has more or less corresponded with actual growth rates. This law also predicts that the classical computer will reach its physical limitations when the number of semiconductors on a single chip reaches the atomic level. The quantum computer opens up possibilities for development at the atomic and subatomic levels, able to process ever larger and more complicated information at faster speeds.

But even for Prof. Itoh, who has decades of experience studying the physical properties of silicon isotopes, quantum computing is still a new and unknown world. So he has devoted himself to the field, attending conferences around the world in the related fields of quantum mechanics and information engineering. He has even toured labs up and down the US West Coast to explore possibilities of isotope engineering. During that tour, he met Dr. Yoshihisa Yamamoto, who was working at Stanford University at the time.

“Dr. Yamamoto gave me some serious homework. He asked, ‘Is it really possible to arrange isotopes individually?’” I later showed that it was at least theoretically possible at an academic conference held in Hawaii. Our research seem to fascinate him.” Prof. Itoh ended up dining with Dr. Yamamoto over the next few days in Hawaii, during which time they developed a new framework for quantum computing . They each took the idea home and after many discussions the resultant paper, An All Silicon Quantum Computer, was published in 2002. “It’s become one of our most cited papers,” Prof. Itoh says.

The quantum computers discussed in that paper utilize the silicon-28 and silicon-29 isotopes. The two isotopes not only have different masses but also have another key difference—something called nuclear spin. Isotopes that have nuclear spin behave like extremely small magnets. Conversely, those without nuclear spin are completely non-magnetic. Silicon-29 is the only silicon isotope to have nuclear spin, but information can only be stored on its atoms after arranging them inside a high-purity silicon-28 host, which creates a magnetic field and allows for control of its nuclear spin. This allows information to be processed tens of thousands of times faster than current classical computers. It is thought that the general knowledge of nuclear magnetic resonance (NMR) can be used to control quantum information.

“I caught a lot of flack from international researchers who wondered why on earth I would study silicon-29 exclusively, but with these new discoveries I think my perseverance has paid off,” says Prof. Itoh. “As of now, there are two methods of silicon quantum computing: one that incorporates silicon-29 and one that treats it as a nuisance and attempts to remove it entirely. In any case, our semiconductor isotope engineering is essential. It's important for us to be cognizant of the accumulation of past research and to integrate it into our newfound knowledge,” he continues.

Underpinning Prof. Itoh’s ideas is his rich knowledge of the physical properties of silicon isotopes, gained over years of research. Prof. Itoh smiles and says, “By 1998, we had conducted countless experiments on germanium, so we had already gained a vital understanding and theoretical knowledge before moving onto silicon.”

Prof. Itoh claims that there are three major reasons for the firm foundation that fostered his groundbreaking research. The fact that they had immediate access to materials, that those materials were first-rate, and that they had a basic understanding of the material’s physical properties.

“I’ve been called to meetings around the world to discuss NMR, surface physics, and information engineering, among other things. All sorts of people are excited about the possibilities of quantum computing. It's not limited to any one field,” Prof. Itoh says.

The world has reached a level of unprecedented accuracy of atom placement in quantum computing—within just a few atoms and electrons. The length of time that quantum information can be held on a quantum bit—whose electron and nuclear spins are delicate and easily scrambled—has increased one million times over, from just a few microseconds to tens of seconds. And this length continues to increase. Major corporations have now begun full-scale investment into applications for quantum computing.

“Soon there will be technologies that depend upon some of the isotopes we are currently researching. We are experiencing a shift from basic research to technology transfer,” says Prof. Itoh, who has ambitions to reach new heights as a researcher and expert in the field. He says that diamonds, which are made of carbon, are in his future. “Silicon quantum bits only work at extremely low temperatures, around -273 degrees Celsius. This is OK for quantum computing which will enable today’s mission impossible. However, some quantum application requires higher temperature operations. In this sense, diamond-based devices is interesting because they can be used at room temperature,” says Prof. Itoh. He explains that a single electron spin located within a diamond can be employed as a magnetic-field, electrical-field, or temperature sensor having unprecedented sensitivity. The quantum state of the electron spin in diamond at room temperature is sensitive to the surrounding environment, making it highly difficult to hold information for quantum computing. But Prof. Itoh is enthusiastic.

“Being sensitive isn’t a bad thing. It means that it could be a great sensor.” He is currently chartering new waters to prove new concepts in quantum sensing—detecting large amounts of information by means of quantum spin—built on expertise in isotope engineering.

The applications of semiconductor isotope engineering are wide-ranging.

“In addition to quantum computing and quantum sensing, we’re also interested in the unavoidable atomic diffusion phenomena in silicon integrated circuit processing,” he says. The thermal processing needed to produce ever-smaller silicon parts in semiconductors can have theoretical effects on performance. Prof. Itoh uses isotopes to examine their physical and chemical behaviors.

“We’ve launched the Technology Computer Aided Design (TCAD) Research and Development Center at Shin-Kawasaki Town Campus (K2), where we currently head up research as a collaboration between academia and industry. Classic computers still require our knowledge of semiconductor isotope engineering, too,” he says with a smile. We asked what is in store for future research into semiconductor isotope engineering, which has an ever-increasing influence on the world. His answer surprised us.

“It may seem strange, but I’ve always wanted to put more effort into education than research. Educating world-class talent is the best way to ensure a sustained driving force behind research and to generate a virtuous cycle.” He not only wants his students to encourage and admonish one another to achieve academic progress, but he also wants them, and his semiconductor materials, to travel beyond Japan to advance international collaboration.

“My lab has a high rate of advancement to doctoral programs. I believe that a true leader and advisor must first gain international recognition before they can successfully educate the next generation of international researchers,” he notes. Recently he seems keen on collaboration with other fields of research. “I’m particularly interested in collaborations between medicine and engineering. As researchers, we have a social responsibility to do research and to do good through that research. I think this is possible through transdisciplinary collaboration despite its known difficulties,” he says.

While we can’t know just yet how the theoretical, experimental, and applied knowledge of Prof. Itoh’s research will connect with other realms of science, one thing is certain—his pioneering journey into the unknown is far from over.

Kohei Itoh

1989 – Bachelor of Engineering (B.Eng.) Instrumentation Engineering, Faculty of Science and Technology, Keio University

1992 – Master of Science (M.S.) Materials Science and Mineral Engineering, University of California, Berkeley

1994 – Doctor of Philosophy (Ph.D.) Materials Science and Mineral Engineering, University of California, Berkeley

After serving as a research assistant and subsequently a postdoctoral research fellow at the UC Berkeley and Lawrence Berkeley Laboratory, Prof. Itoh returned to Keio as a research associate at the Department of Applied Physics and Physico-Informatics within the Faculty of Science and Technology. He became Assistant Professor in 1998 and was promoted to Associate Professor in 2002. He has held the position of Full Professor since April 2007. He was elected Dean of the Faculty of Science and Technology and the Graduate School of Science and Technology in April 2017.

In addition to his duties at Keio, Prof. Itoh has served as a PRESTO (Precursory Research for Embryonic Science and Technology) researcher, competitively selected by the Japan Science and Technology Agency. He has been a lecturer at the former Graduate School of Natural Sciences at Chiba University, Project Professor and Visiting Professor at The University of Tokyo, and Visiting Professor at the National Institute of Informatics. He has also served as a member of Science Council of Japan (SCJ) since 2011.

He received the Japan IBM Prize for Electronics in 2006 and the Japan Society for the Promotion of Science (JSPS) Prize in 2009. He was awarded as an Applied Physics Fellow in 2015 by The Japan Society of Applied Physics.

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