Researching high-performance qubits for quantum computers using "floating electrons" on liquid helium and solid neon

Q1. What inspired you to become a researcher?

A1. Being struck by Newton's laws and quantum mechanics

When I first came across Newton's three laws of motion in my high school physics textbook, it changed the way I saw the world. I was fascinated that a few simple equations could describe natural phenomena and get to the heart of how things work. At university, I read biographies of scientists. I came to admire them, and I began to think of research as a career.

Learning quantum mechanics affected me even more deeply than that high school experience. It gives a precise description of phenomena that seem almost unbelievable by everyday intuition, and yet these are the rules that govern our world. What really drew me in was that you can set emotions and preconceptions aside and reach the truth through careful, step-by-step logic.

For my third and fourth years of university, I studied in France because I wanted to see more of the world. While I was there, I did an internship in Italy. That was where I first learned quantum information theory and became interested in quantum computers. After returning to Japan and finishing my master's degree, I started a PhD at Delft University of Technology in the Netherlands. Researchers from all over the world were gathered there to develop many different kinds of quantum computers, and in that cutting-edge environment I threw myself into research on qubits made from electrons in semiconductors.

Q2. What are you working on now?

A2. Turning electrons floating in vacuum into qubits

Electrons can be made to float in the vacuum just above the surface of liquid helium or solid neon. These are called "floating electrons." I am using them to build high-performance qubits, the basic building blocks of quantum computers. Impurities and defects create noise that disturbs a qubit and destroys its quantum state, but in a vacuum these disturbances are greatly reduced. During my PhD on electrons in semiconductors, I realized that defects and impurities can't be avoided even in very high-quality silicon crystals, and that removing their effects takes an enormous amount of time. That is what turned my attention to electrons in vacuum.

Qubit technologies fall into two broad groups. Solid-state qubits, such as superconducting circuits and semiconductors, are easy to integrate on a chip, but noise from their surroundings makes it hard for them to hold their quantum states. Qubits in vacuum, such as trapped ions and cold atoms, can hold their quantum states for a long time, but they are hard to integrate. The biggest strength of floating electrons is that they combine the advantages of both. Until recently, though, there was no way to measure the quantum state of electrons in vacuum electrically, and that is essential for integration. So we developed a new technique that uses resonators to read out quantum states electrically. This was a major step toward making qubits from floating electrons.

We are currently working on both liquid helium and solid neon at the same time. Each comes with its own challenge: in solid neon, it is hard to control where the electrons sit, and in liquid helium, the surface fluctuates. While working out which of these challenges we can overcome, we are also developing low-power microwave oscillators that run at cryogenic temperatures, which are needed to control and read out the qubits.

Q3. What message do you have for aspiring researchers?

A3. Enjoy the moments when nature defies your intuition

Don't assume that where you are now is all there is. Go out and explore the wider world. As an undergraduate I studied in France and did an internship in Italy. I then did my PhD in the Netherlands and my postdoc in Okinawa. Every move broadened my perspective, and those experiences are the foundation of my research today.

In research, you sometimes get results that are completely different from what you expected. When that happens, it's important not to cling to your assumptions. Work through the equations carefully, one by one, and think about it scientifically. For me, the best part of research is the moment when a result goes against intuition — and that is exactly what makes it fascinating.