The 2017 Sir Martin Wood Prize was awarded to Dr. Michihisa Yamamoto, now at RIKEN, for his successful observation of quantum phase detection. This prize was established in 1998 to promote scientific exchange between the UK and Japan, and to recognise the achievements of outstanding young Japanese researchers.
In this blog post, Dr. Yamamoto explains how he started his award-winning research on quantum phase detection of electrons and the quantum mechanical view of the wave function that led to his successful observation. Dr. Yamamoto also explains his childhood interests, how he refreshes his mind to make way for new observations, and secrets about his research.
"We are working on the construction of a qubit system based on the interferometer. The use of propagating electrons has the advantage that a large number of qubits can be controlled with small hardware."
How did you start your research on quantum phase detection?
Experiments on detecting the quantum phase of electrons had already been done all over the world long before I started this research.
In Japan, Dr. Shingo Katsumoto and Dr. Kensuke Kobayashi of the Institute for Solid State Physics at the University of Tokyo had done excellent research relating to the Aharonov-Bohm (AB) interferometer, and I was greatly inspired by their work when I was a graduate student. Shortly after I finished my doctoral course, my supervisor Dr. Tarucha asked me, "Can you do something quantum information-like, with quantum wires?”
From then, I came up with the idea of a new quantum interferometer that combines an Aharonov-Bohm (AB) interferometer connected with quantum wires by tunnel junction (enabling electrons to travel back and forth). In this interferometer, a qubit can be defined by which wire the propagating electrons reside in.
At the time, my idea of a quantum interferometer seemed so natural that I remember wondering why other researchers hadn’t done similar experiments with it. The phase measurement results of other groups were inconclusive, which made me realise the interferometer I had produced was the only one able to achieve precise and correct phase measurements. I still feel very lucky to have made this discovery!
Can you explain the role of the AB interferometer?
It’s an interferometer for electrons in which the interference of the electron wave changes according to the magnetic flux surrounded by the path of the electron propagation. The interferometer I have developed is designed so that the magnetic flux is surrounded by two paths (quantum wires). The electrons are not allowed to pass through or reflect back in any way other than the two paths. It’s a two-path interferometer, similar to the famous double-slit experiment. The current component that oscillates as the magnetic flux changes (AB oscillation) is due to pure quantum interference.
How did you succeed in controlling and measuring the phase of electrons?
In the interferometer, the phase component of the electrons was extracted using AB interference (as the phase of AB vibration). Myself and the team then confirmed that the phase can be controlled by voltage.
The interferometer works as designed, but adjusting the device can be challenging in its own way. It took us a while to get confirmation, but once adjusted, the phase could be measured much more clearly than expected - I was very impressed! The level of precision and reliability of the phase measurement was completely beyond the research that had been reported up to that point.
However, there were some hurdles to jump at first. When my collaborator tried to send the interferometer sample to a researcher overseas for measurement, the graduate students were unable to adjust it properly and the joint research was aborted. A person is required to take the quantum mechanical spread of the electron wavefunction into account to be able to tune it, but once this concept is understood, it can be easily adjusted. The accuracy of the phase measurement is quite high - it’s accurate enough to capture changes in the path length of an electron in a semiconductor of less than one nanometer.
What are some of the applications of this interferometer?
The fact that we can see the phase of the wave function directly, which is the most fundamental physical quantity of quantum mechanics, is interesting in itself! It’s easy to imagine that many researchers who have attempted this research have been excited by the prospect of observing the phase change of electrons propagating in solids. The observation of the phase provided many fundamental insights into quantum mechanical scattering of electrons. It even has potential applications in quantum information processing, since a qubit can be defined by a propagating electron.
What research are you working on now?
We are working on the construction of a qubit system based on the interferometer. The use of propagating electrons has the advantage that a large number of qubits can be controlled with small hardware. We are also trying to understand the fundamental physics of coupling of distant electron spins using electron wave control.
What brings you the most joy in your research?
I'm an experimentalist, so when I see what I want to see - and it's the first time in the world - that's when I get excited! I’m excited from the moment I think I can see it. I also enjoy it when unexpected results come out because it leads to new knowledge. I regularly fantasise about new experiments and it may seem surprising, but I am often a little frustrated, while very impressed, when other people conduct brilliant research - because it’s one less thing for me to uncover! It’s a pleasure for me as a researcher to be able to personally interact with wonderful researchers around the world.
What were you interested in when you were in school?
When I was in elementary school, I was so absorbed in moving my body and playing the piano that I rarely studied at home, except for homework! The only scientific thing I did was catching bugs. When I read in a book that the planets revolve around the sun and that time moves differently depending on the location, I believed there was a small universe in the invisible dust in front of me (countless invisible stars revolving around each other), containing people living their lives, just like us. I thought about a larger world outside of our world where time moves slower, and that our world could suddenly be destroyed. I always wanted to know more about the small invisible world.
How do you refresh your mind?
I used to play the piano, but since having children, I don't play much anymore! I used to prefer to play music and partake in other activities alone, but as I've gotten older, I've realised that I can refresh my mind by talking with someone.
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