Converting Information into Free Energy – Proof of “Maxwell’s Demon” and Development of Information Thermodynamics – #AJ branch-off edition


“AJ branch-off edition” is a category of articles which past published in “Acaric Journal”, a career magazine for graduate students and researchers published by Acaric Co., Ltd., or fresh articles only available on the web. This time, we bring you the articles from vol.1.

 Maxwell’s daemon” was a thought experiment conducted by Maxwell at a time when atoms and molecules had not yet been discovered. It was a concept that linked information and thermodynamics, using information to control the energy balance. We asked Dr. Sagawa, who has demonstrated this idea together with Dr. Toyabe, what kind of study quantum information theory is, and what kind of path students who study this kind of study will take.

– Please tell us how you became interested in quantum information theory.

 When I was in my third year of undergraduate school, my friends and I read in turns a book on quantum information by Nielsen and Chang in a voluntary seminar. That was the beginning.

– We know that you are famous for your paper about “Maxwell’s Demon”, what is it about ?

 It’s about the connection between information and thermodynamics. The second law of thermodynamics determines the minimum amount of work required to process information, and in particular, it involves what is called the law of increasing entropy. Since information is a form of entropy, it is essentially the same as the entropy of thermodynamics, and in that sense, information and thermodynamics are linked. In this sense, information and thermodynamics are linked. We can determine the principle, physical limit of how much energy is required for information processing. This is the basic idea that links information and thermodynamics.

“Maxwell’s Daemon” is Maxwell’s way of pointing out that we use information to get work done. The modern view is that information can be used to control the energy balance.

 It is difficult to prove this with atoms and molecules, but I tried to prove this “Maxwell’s daemon” with colloidal particles. This was in 2010, when I was in my third year of the doctoral course, and it was the first experiment to realize Maxwell’s daemon.

– What exactly do you mean by using information?

 If you have two rooms filled with gas, and if a fast molecule comes from the right, you open the door in the middle of the room, and if it comes from the opposite direction, you close the door. As a result, only molecules with high velocity (high temperature) can be collected on the left side. By looking at the information about how fast the molecules are moving and controlling them, we can create a temperature difference and reduce entropy. In the 19th century, we did not know if atoms and molecules really existed, so Maxwell used thought experiments.

– So the hypothesis of the 19th century thought experiment was able to be experimentally created in 2010. Were there several of you working on it when it was realized?

 I was a theorist and Dr. Shoichi Toyabe, who is now an associate professor at Tohoku University, actually conducted the experiment. I myself had developed various theories, but Dr. Toyabe’s technology was a good fit and made it possible.

– How did you think about your career while pursuing your research?

  I have loved physics and mathematics since high school, and I was planning to go on to graduate school. Around the summer of my first year of Master course, I got my first results and wrote a paper about it, which attracted a good amount of attention, so I thought I could continue as a researcher, and here I am.

– It seems that you had an idea of the form that would lead to your current research at a very early stage. Did you start your research when you were an undergraduate?

  I started my research in graduate school. I was interested in both quantum information theory and statistical mechanics, and I chose Professor Masahito Ueda’s laboratory where I could study both. One day, my professor suddenly said to me, “Aren’t you interested in Maxwell’s daemon?” That was the trigger.

 At that time, the modern theory of statistical mechanics and quantum information were gaining momentum, but there was no one in the world who wanted to rethink Maxwell’s daemon by combining them. I think Professor Ueda had a good point of view.

Quantum computers are attracting a lot of attention and are becoming a commonly heard term.Do you have any thoughts on this situation?

 When I was a master’s student, there were probably only a few people who really thought that quantum computers could be developed. Dr. Yoshihisa Yamamoto, who has been leading the field of quantum computers in Japan, held a summer school in Okinawa around that time, and we often discussed the feasibility of quantum computers. Since then, the field has advanced rapidly with the entry of Google and IBM, and I feel that times have changed.

– Where is Dr. Yoshihisa Yamamoto now?

  He is now the director of the PHI Lab of NTT Research, Inc. which was established in Silicon Valley. He used to be at NTT in Japan. Many researchers have come from there.

– In that generation, were companies more advanced in their research?

 The same superconducting quantum computer used by Google was built by Dr. Yasunobu Nakamura, who is now in the same department as me, when he was at NEC.

– We have heard that many students in your laboratory go on to work for companies.

 Many of them go on to work in the private sector in IT-related fields. Programming skills are immediately useful, and I think that mathematical ability is highly valued. There are also students who are educational YouTubers.

– What is the percentage of students in your lab who go on to doctoral studies?

 About half. The rate is relatively high in the Department of Physics and Engineering.

– We’ve heard some students say that theoretical physics and mathematics are too basic research and that private companies won’t pick them up.

 This is not true, at least not for theoretical physics. There are companies that can do basic research on quantum computers, and I don’t hear of anyone around me who is having trouble finding a job.

– That’s very reassuring. We think that there are people who are confused about that.

 In particular, foreign companies often give preferential treatment to those who have a doctorate, so you have more options if you have a doctorate. If you are ambitious, I think you should get the Ph.D. Ph.D. doesn’t have many negative effects, and everyone is able to go where they want to go. That is my feeling.

– What are the future plans for the research areas you are working on?

 I am particularly interested in developing information thermodynamics. It is a theme that is relevant to many different fields. In particular, I would like to focus on quantum many-body systems. For example, I would like to create a heat engine that uses quantum effects. I would like to think about information thermodynamics in quantum systems.

 One more thing, although it is not related to quantum, research on the application to living organisms is popular all over the world. In Japan, there is a Grant-in-Aid for Scientific Research (KAKENHI) project called “Order and Design Principles of Life Unraveled by Information Physics.

– Finally, could we have a message for students?

 I think that the master’s years are a good time to work on themes that are really interesting to you and that you think will be at the forefront of the world, rather than thinking about your career or what you think will be popular if you do this. Since we are at an age when we have relatively more freedom, I think it is a good time to think about what is really interesting to us, apart from what is going on in the world. That’s what I want to keep in mind. It’s a bit idealism, but…

So you want students to think about what they really want to do and paint a big picture. Thank you very much.

Profile (at the time of the interview)

Dr. Takahiro Sagawa

Born in 1983 in Hyogo, Japan. He completed his doctorate in physics at the University of Tokyo in 2011. Ph.D. in Physics. He is currently a Professor at the Department of Physics and Engineering, Graduate School of Engineering, the University of Tokyo from 2020. He specializes in the theory of quantum information and statistical mechanics.



  • 理系・院生特化のスカウト


  • 分野別の特別イベント


  • プロに就活相談