Quantum-Mechanical Optoelectronics
from Keio's Faculty of Science and Technology New horizons of optoelectronics opened up
by quantum mechanics.

Applying principles of quantum mechanics to semiconductors and photonics

“Quantum-mechanical informatics is a focus of attention today as the greatest application of quantum mechanics. Effective and advanced use of quantummechanical characteristics enables many wonderful things that conventional informatics couldn’t even imagine previously – things, such as quantum teleportation capable of communicating information to remote places in an instant; a quantum computer that can solve extremely demanding tasks in an instant via super-parallel computing – a task that would take hundreds of millions of years for currently available computers to perform; and a quantum cryptographic system that is absolutely safe against eavesdropping,” remarks Dr. Hayase.
Dr. Hayase’s specialty is quantummechanical optoelectronics. The focus of her studies is on systems based on a combination of pulsed light and nanostructured semiconductors that can freely control quantum-mechanical characteristics of photons* and electrons.
“Today our information-oriented society is supported by optoelectronics, but it’s far from perfection in terms of effective application of the principles of quantum mechanics. If we succeed in making the most of quantum-mechanical characteristics, it would be possible to create totally new technologies that revolutionize our common knowledge. One such example is quantum informatics,” she adds.

As you may know, all information we handle on our PCs, cellular phones, the Internet, and so on is represented by sequences (“01101 …”) of binary numbers of “0” or “1” with a “bit” as the minimum unit (see Fig.1). It is the intensity (ON/OFF) of light or electric current that expresses “0” or “1.” However, this technology merely constitutes a part of the characteristics of light and semiconductors. Therefore, scientists around the world are expecting much of quantum-mechanical informatics. The truly wondrous phenomena of “0 can be 1 simultaneously” that can take place in the world of quantum mechanics are said to enable a new horizon of informatics no one had ever dreamed of previously.
“It is the famous ‘Schrödinger’s Cat’ experiment (Fig.2) that well illustrates the concept of quantum mechanics. In this experiment, a cat is put in Chamber A in which a poisonous gas generating device is set up. On the other hand, with Chamber B, a randomly operating switch is connected to it. When the Chamber B switch is turned ON, the gas generating device in Chamber A is actuated. Until one looks into either one of the chambers after a certain lapse of time, it is impossible to know whether the cat is alive (0) or dead (1). This state is referred to as “quantum superposition” – a state where 0 can be 1 simultaneously. A quantum computer uses this superposition as a unit of information (quantum bit). Since one quantum bit is capable of 0 and 1 processing simultaneously, numerous quantum bits enable super-parallel computation.”
Another important concept is “quantum entanglement.” The cat is alive (A=0) before the switch is turned ON (B=0), while it is dead (A=1) after the switch has been turned ON (B=1). In other words, it is either “if A is 0, B is 0” or “if A is 1, B is 1.” But you cannot tell which is which until you actually observe it. This state is known as quantum entanglement. If Chambers A and B are placed at separate locations, the state of Chamber B becomes definite the moment Chamber A is observed, making it possible to communicate information between two remotely distant locations. This is the principle of quantum teleportation. 　More than one quantum state, such as superposition and entanglement, remain coexisting possibilities until being revealed. Once revealed, a particular quantum state becomes definite. In other words, this means that quantum states as possibilities are broken once you observe them.
“The photon is useful for cryptography partly because it is the smallest particle that cannot be further divided. It is also because eavesdropping can be easily revealed since quantum states fail if someone attempts to eavesdrop (= observe) on a particular piece of information. Furthermore, the ultimate in energy-saving telecommunication is possible because quantum-mechanical telecommunication works with only an extremely small amount of energy,” Dr. Hayase added.

Working energetically to achieve quantum optoelectronics, Dr. Hayase is currently engaged in experiments – exchanging quantum states between semiconductor electrons and photons, controlling these states, and so on (Fig.3). However, numerous problems are encountered to convey quantum - mechanical information to semiconductors.
“A quantum state is so fragile that it can be rapidly broken. Therefore, information needs to be controlled and transferred while maintaining the quantum state, which is extremely difficult.”
Coming to the fore as a possible solution are semiconductor quantum dots – a type of semiconductor suitable for maintaining and controlling quantum states. Quantum dots are ultrafine particles of semiconductor whose size is 10-8 meter. With this semiconductor, electrons can be confined in a very small area, making that area work as if it were an atom. This makes it easy to control and maintain the quantum state of each one of the electrons.
“What’s outstanding about quantum dots is that their characteristics can be controlled freely according to their size and shape. My research team employs a special technique to produce quantum dots that can interact strongly with the light used in optical fiber communication. We have also succeeded in retaining the state of superposition much longer than those developed by other groups.”
That said, it is unavoidable that quantum states are constantly being broken every moment. So, as an ultrafast flash, Dr. Hayase uses ultrashort pulse laser capable of emitting light instantaneously in an incredibly short period of 10-13 second. This enables her team to make use of nonlinear phenomena that cannot take place under ordinary light conditions, which in turn makes it possible to control superposition and quantum entanglement.
Given the extremely weak intensity of light that corresponds to one photon, it is very difficult to cause intended effects between photons and electrons. So Dr. Hayase is looking for a method of collecting many quantum dots that could enhance interactions between photons and electrons.
“What makes quantum mechanics intriguing is that it has virtually unlimited potential. I’m tackling research activities with the hope of achieving a fantastic discovery that could revolutionize the world in decades ahead.”

Though I have little memory of my childhood, my parents often said that I had been a very inquisitive girl always asking “why?”, but I’m sure I was not particularly good at science.
I was born in Fukushima Prefecture where everything is easy and slow. So I remember that I didn’t study hard as an elementary schoolgirl. In my junior high school days my family moved to Saitama Prefecture and I went on to a prefectural senior high school. An encounter with a physics teacher was inspiring and marked a significant turning point in my life.
Speaking of high-school physics classes, it’s usually the case that students mostly tackle the textbook and solve numerical calculation-oriented problems rather than making physical experiments. But my teacher was different. In our physics class, conducting an experiment was an integral part of study, which made students think about “why.” When it comes to regular examinations, the teacher rarely gave calculation-oriented problems but instead posed questions like “Describe why so.” As a thinking type of student by nature, my encounter with this teacher awakened my interest in physics. That said, becoming a researcher still remained an almost unreachable dream for me back in those days.
This is because as a member of my high school’s rowing club, I had spent most of my time for club activities including participation in the inter-high school rowing competition. Up until September of my third year in high school (when the National Athletic Meet was held), I had literally devoted myself to club activity. So I studied little to prepare for university entrance examinations. Instead, I was admitted via recommendation to the Department of Physics, Sophia University.

When I was a senior at my university, I decided to join the lab of Professor Kazuhiro Ema who was then only in his second year of arrival in his post. The lab was focused on nonlinear spectroscopy using ultrashort light pulses – a forefront field of study leading the world. Until the end of the year as a junior, I had learned theories and experiments the answers of which are known and provided. But at the Ema lab we took up the challenges of experiments no one in the world had ever conducted, and could produce innovative results. Since the lab was only in its second year, we had many things to do – improving the facility, setting down rules, making experimental devices, and so on. But doing all these things was both challenging as well as very fulfilling. We often carried out experiments all night long, but it was not troublesome at all. Once concentrated on something, I’m not satisfied until I carry it out to the very end.
Having put myself in such an environment, my yearning to become a researcher must have grown stronger and stronger. People often say that I appear to be a quiet type at a glance, but I find myself rather stubborn and uncompromising once I’ve decided to do something (laughter). My parents advised me, saying “It’s best to find employment with a company after graduation and marry a suitable young man.” But I gave no ear to their advice and decided to go on to graduate school for a doctoral course.

I’m now in a position to preside over a laboratory and guide students myself. As such, I can say my experience of having learned under Professor Ema is truly useful. Recalling those days, I don’t remember being severely scolded by Professor Ema. He always encouraged his students and helped them develop their potential by praising good things about each student and giving them advice and research themes that could draw out their motivation. I’m modeling my approach toward students after Professor Ema’s.
After earning my doctor’s degree, I joined the RIKEN institute to become an academic research scientist. It was a position as a postdoctoral fellow with a term of three years. I applied for RIKEN by proposing a research theme of my own, which was luckily adopted by the institute. The theme is a bit different from my current theme, but had things in common in that it would handle light and nanostructured semiconductors. Following my service with RIKEN, I moved to the National Institute of Information and Communications Technology (NICT), where I encountered quantum dots. It just happened that at NICT there was a research team engaging in the making of characteristic quantum dots, which I found intriguing. I belonged to NICT for about four and a half years. With the NICT as well, I obtained a position for myself through job-seeking activity. In our academic world, the acquisition of a doctor’s degree doesn’t necessarily mean a stable employment without term.
Striving under such circumstances, therefore, I approached one research institute after another to obtain necessary research funds and secure a stable research environment. During my service with the NICT, I was successfully chosen as an eligible researcher by the Japan Science and Technology Agency’s (JST) “PRESTO (Sakigake in Japanese)” competitive research funding system that targets individual research projects, surmounting a highly competitive ratio of one in 15 applicants. What was particularly good about the “PRESTO (Sakigake)” system was that the adopted researchers and advisers on the screening side get together in semiannual boarding sessions to engage in discussions in an unrestricted atmosphere. On these occasions, the participants make presentations on the progress of their respective research projects. Each participant is subjected to unreserved opinions and criticism from others – a coveted opportunity for obtaining an extremely high level of advice, which provided me with nourishment for future growth. Being a boarding session, nighttime off the core time discussions was also fruitful and enjoyable as we could talk about each other’s future frankly as late as 2 or 3 o’clock in the morning. In the academic world, human relationships are highly valuable. The fellow researchers I could get acquainted with through the “PRESTO (Sakigake)” system are now a great asset for me.
Using participation in the “PRESTO (Sakigake)” system as momentum for stepping up my career, I then became a teacher for the University of Electro-Communications by taking advantage of thetenure-tracking system (a system that encourages young researchers to accumulate experience as independent researchers under employment with a term and obtain stable employment afterpassing rigid screening). And last year, at last, I could find a permanentposition at Keio University. Inretrospect, I can say that my experience as a postdoctoral fellow with a term, though unstable, was a fruitful one because I could devote myself to research. Now I’m in a position to operate my own lab, with six students, undergraduate and graduate, under my care. I feel great responsibility for this duty, which is fulfilling as well.

Since I’m usually leading a restless life, I try to enjoy a relaxed lifestyle when I’m not working, going shopping with my husband, taking trips and so on. Proper rest and relaxation are definitely necessary because research work demands physical strength and energy as well as power of concentration. Likewise, I’d like my lab students to lead a life punctuated with rhythms of hard work and relaxation.