2025.4.3

Recently we wrote an essay. This essay is written for 2025 Gravity Research Foundation Essay Competition. Since it contains less equations than our normal papers, it could be that it is more readable than a regular paper...? Anyway you can see arXiv:2504.01625. The key tool is genuine multi-entropy (GM^{q)), which contains q-partite entanglement but vanishes for all a-partite entanglement where a < q. For the people who are familiar with quantum information a bit, you can think of GHZ state = |000> + |111> for 3 spins ABC. This state contains genuine tri-partite entanglement between 3 spins, ABC, but does NOT contain any bi-partite entanglement between 2 spins, such as between A and B, etc. (It contains bi-partite entanglement between A and BC, but not between A and B. There are no bi-partite entanglement between 2 spins. If this does not make sense, you can trace out C spin and check that you obtain the reduced density matrix rho = 1/2 (|00><00| + |11><11|) for AB. This density matrix for AB is the ensemble of the product state. Thus there is no bi-partite entanglement between A and B.) Our GM^(q=3) measures how much such a genuine q=3 partite entanglement contribution has for a given state. Let me give you another example: let us consider the triangle state, where you have 6 spins A1,A2,B1,B2,C1,C2. A1A2 forms one subgroup, and B1B2 forms another etc. The triangle state is a product state of Bell pair of (A1-B1) x (B2-C1) x (C2-A2). Since this triangle state contains only 2 spin entanglement, but not 3 spin one, our GM^(q=3) vanishes for this triangle state. This GM is first introduced in arXiv:2502.07995.

2025.2.19

From today, we will have a new class: Introduction to Quantum Theory of Black Holes and Holography (II). This is, of course, a continuation of the (I) course of the same name I did last semester, and I will talk about more advanced content this semester. What I am planning to explain in this and next week are;
* Haar random state
* The detailed derivation of the Page curve using the Haar random state
* "Feynman diagram" of these
* Tri-partite states (GHZ-state, W-state) and (genuine) multi-partite states
* What does entanglement entropy represent?
* Various other entanglement measures, such as (logarithmic) negativity, multi-entropy, ...
* How to evaluate these new measures explicitly using Feynman diagram

Some of them are in my recent papers. So if you are interested in these, join my class!

2024.12.20

This week was the last week of my classes: [Introduction to Quantum Theory of Black Holes and Holography (I)]. And today, we had our end-of-the-year Annual Review Meeting at 饗食天堂 新竹店. The food there is really good.
Since I last posted, a lot has happened. On Oct., we had an international conference Quantum Extreme Universe: Matter, Information, and Gravity at OIST, Okinawa, Japan. And on Nov., I was invited to the international workshop 2024 East Asia Joint Workshop on Fields and Strings at Kaohsiung, Taiwan, which is organized by my friends in Taiwan. I gave a blackboard talk on 1/D Dissipation in large N matrix models which I have done with Dan and Takanori. This was actually my first time giving a blackboard talk at an international workshop, but I think it went pretty well, except that I went a little over time. :-)
I also recently wrote a new paper arXiv:2412.07549 on multi-partite entanglement of an evaporating black hole and its Hawking radiation. In this paper, we draw a very interesting new curve, which we named as ``multi-entropy curve''. This curve is a natural generalization of the famous Page curve. The Page curve is for bi-partite systems where we evaluate entanglement entropy between an evaporating black hole and whole radiation. On the other hand, the multi-entropy curve is for multi-partite systems by dividing the Hawking radiation into finer subsystems. I will probably write more about this paper next year. Anyway, this work is done with my wonderful collaborators, Mitsuhiro and Simon. I thank them for the wonderful collaboration. Next January, we will organize an international workshop Recent Developments in Black Holes and Quantum Gravity at Yukawa Institute, Kyoto University. Both of my collaborators will give a talk there. So if you are interested in, please join the workshop which must be exciting.
Finally, I wish you all a happy Christmas and a happy New Year.

2024.10.10

Today is a holiday in Taiwan so I update my homepage. I published a new paper this week on arXiv, arXiv:2410.04679. This collaboration started with a discovery by my wonderful collaborator, Dr. Mitsuhiro Nishida. He pointed out to me that the IOP matrix model, arXiv:0808.0530, an old matrix and vector model that we have studied in the past for the black hole information problem, has the same structure as the famous PSSY model (or the West Coast model), arXiv:1911.11977 at the planar level (in the sense of 't Hooft's large N limit). The PSSY model is a very nice toy model of 2D quantum gravity that reproduces the Page curve, which is key to the black hole information problem. Since one can conduct the gravitational path-integral in the PSSY model, one can show explicitly that the dominant topology of spacetime changes when exactly half the degrees of freedom are emitted from a black hole as radiation. The analysis is done at the planar level, but it is also interesting to consider the quantum gravity corrections in the PSSY model, which corresponds to 1/N corrections. Quantum corrections to the IOP matrix model have already been calculated in our old paper, which is tedious. Therefore, in this paper, we show that the quantum corrections to the PSSY model can also be calculated using the correspondence between the two models. These are our main results. The corrections are tiny, of course, but it is still interesting to calculate these. The most important lesson I can tell from this project is that it is important to have a wonderful collaborator :-)

2024.09.19

My website has moved to NTHU/National Tsing Hua University. My new course on quantum black holes started at NTHU. See Teaching/Classes | 授業 for more course information. Things are busy at a dizzying pace, though, I also submitted my first paper to arXiv after coming here, that is arXiv:2409.05981. It is about thermalization and dissipation in many matrix models. Matrix quantum mechanics is very hard to solve in general, but this is a key point for black hole thermalization and dissipation in a holographic dual viewpoint. We attack this problem by pointing out a very interesting and new 1/D expansion. In addition, last month, I was invited to an international conference at ICTS, TIFR, Bangalore, India, and gave a talk there about the IP matrix model and Krylov complexity. The conference is titled as ``Quantum information, quantum field theory and gravity'' and its website link is this. I was satisfied because the audience response was very positive. The international conference in India made me nostalgic for the people I had not met for a long time and the unique Indian atmosphere that had not changed. Amazingly, it's been a decade or so since I last visited India. The months go by so fast. But from now on I would like to visit India more often to discuss physics with my friends there.

2024.07.30

I will retire from Osaka University, where I have worked for 10 years, at the end of July 2024 and move to National Tsing Hua University, Taiwan, as an associate professor from August 2024. At the same time, I will also hold a position at the Yukawa Institute, Kyoto University, as a (specially-appointed) affiliate associate professor.
(in Japanese)
10年間お世話になった大阪大学を2024年7月末で退職し、2024年8月から台湾の國立清華大学物理学系に副教授として移ります。 同時に、京都大学基礎物理学研究所も基研特任准教授として兼任いたします。

2023.11.18

Time flies very fast. I went to Osaka Prefectural Yao High School to give a "delivery lecture" yesterday. The content of the lecture was "Thought Experiment and the Universe". Starting with Galileo's thought experiment, I discussed Einstein's thought experiment, time and space, black holes, and emergent space-time. The students were energetic so I enjoyed it. I also went to Taiwan the week before to attend this Taiwan string workshop. It was hot in Taiwan. While I am very happy that most of the papers I published this year were accepted by JHEP, I am now in the process of finishing my next work.
(in Japanese)
時間が流れるのは非常に早いですね。昨日は大阪府立八尾高校で「出前講義」をしに行ってきました。講義の内容は理論物理学の最前線：「思考実験と宇宙」です。ガリレオの「思考」実験に始まり、アインシュタインの思考実験から時間と空間、そしてブラックホールと創発する時空まで駆け足で話してきました。生徒さん達も元気がよく、私も楽しむことができました。また先々週には台湾のこの研究会にも行ってきました。日本は最近寒くなってきましたが、台湾は非常にあたたかい、いやむしろ特に南部の高雄などは非常に暑かったです。今年出した論文のほとんどがJHEPにアクセプトされ非常にめでたい中、現在、続編の仕事をいま全力で仕上げている途中です。

2023.9.8

I have been very busy recently, visiting the Kavli Institute in Beijing at the end of August and organizing a workshop with various local people in Beijing. There were various interesting talks related to thermalization, SYK, Black Holes, etc and I enjoyed very much many of the contents, the physics, and the discussions there! China was vibrant and very nice, and I would like to go back in the future!
As soon as I returned, from this week I had another visitor from India, Prof. Sandip Trivedi, who has been my friend since I was a post-doctoral fellow in India a long time ago. It was very fruitful to discuss various physics (IP model, SYK model, JT gravity, etc.) with him and Sunil, who is currently in Japan, and to discuss the direction of my research.
I am going to Kyoto next week for this workshop, which I am one of the organizers. I will talk about the Krylov complexity of the IP matrix model with Dr. Nishida on the 11th of September, so if you are interested, please come or see my slides.

2023.7.12

Last week I attended a very fruitful workshop in APCTP, Korea, Quantum Black holes, Quantum Information and Quantum Strings. I had a very useful discussion with various researchers from around the world. I thank the organizer for the wonderful workshop and invitation! At the meeting, I talked about this recent paper on arXiv arXiv:2306.04805 which is in collaboration with Dr. Mitsuhiro Nishida, a former grad student at Osaka U. I have also been very busy recently and have not had time to upload my website, but I will be posting various papers that I have recently completed here shortly.

2023.5.15

Today I am excited to announce our new paper appearing in arXiv arXiv:2305.07505! This is the work that I have done with my wonderful collaborators in TIFR, India(=where I did a postdoc a long long time ago), Arka, Sandip, Sunil(=now Sunil is a postdoc here in Osaka) and with my student Takanori. It investigates the relationship between sparse random matrices and gravity. You may have heard of the SYK model (=Sachdev-Ye-Kitaev model). This model is made up of N Majorana fermions. The important thing is that the coupling constants in this model are random numbers, and because of that, this model allows a new large N expansion, which is similar to the famous 't Hooft's work. Anyway, the fact that the coupling constants are random numbers implies that the SYK model itself is sparse random matrices. Here sparse implies that some elements of the random matrices are removed. In fact, in the SYK model, we have q fermion couplings and if we sum over all q, and if all the variance of the coupling constants are the same, it is a random matrix called GUE. Given this, we have studied the vast sparse random matrix "landscapes", which includes the SYK model and also the "local" SYK model. The local SYK model is the model where the q couplings are allowed if and only if they are next to each other. Our diagnostic tools to study this vast sparse random matrix landscape include density of states, thermodynamical quantities, normal correlators and also out-of-time-ordered correlators, nearest neighbor level spacing, etc. It is more than 80 pages and contains not only many beautiful figures but also a very nice appendix such as sparse SYK model analysis. So please have a look at it!

2023.5.1

Our new paper arXiv:2304.14620 appears today! We studied holographic complexity in the accelerating expanding universe under the presence of small and early perturbations. Very early perturbation becomes a shock wave, traveling through space and time and we found that under this shock wave, the hyperfast property of de Sitter complexity is always delayed. It turns out the other group in Israel and Canada also studied the same problem and obtained similar conclusions (but with much more detailed) in arXiv:2304.15008. I also received several interesting comments from experts. de Sitter spacetime is interesting since it is related to our universe and we hope we can understand it better in the future!

2023.4.8

The international conference in Kyoto, `Quantum Physics of Black Holes 2023' was a success! There were so many interesting talks and we thank all the speakers and participants, especially speakers from abroad!

2023.3.27

I gave an informal talk on de Sitter spacetime and complexity at Nagoya U. today. Thank you so much for all the interesting questions!

2023.3.18

Another new paper arXiv:2303.10314 appears today! We study the late-time behavior of the spectrum form factors in topological gravity, which includes JT gravity. Never heard about topological gravity? Me too! Indeed, I had never understood that either, until last fall, when Prof. Sakai kindly explained to me all about it very clearly at the 1st study group of black holes and quantum gravity (the website is in Japanese). We thank Sakai san and Okuyama san for their wonderful collaborations. This project was very educational for me!

2023.3.10

Our new paper arXiv:2303.05025 appears today! A simple message in this paper is that, unlike Einstein gravity in higher dimensions, in 2D gravity with dilaton (=a scalar field), a vacuum solution is characterized by both dilaton and metric. Therefore by ignoring the dilaton, if you determine the quantum complexity only from the metric i.e., complexity = volume conjecture, it is not the right prescription. You should take into account the dilaton as well, especially for de Sitter spacetime (=expanding universe).

2023.3.7

Today we have exciting news. Takanori Anegawa (姉川尊徳) received `Outstanding Research Award, Graduate School of Science, Osaka University'. It seems that only one person from the Department of Physics in grad school received this each year. Congratulations Takanori! We are very proud of you.
(in Japanese)
大阪大学大学院理学研究科優秀研究賞： 姉川 尊徳君(大阪大学理学研究科)が大阪大学大学院理学研究科優秀研究賞を受賞しました！こちらも参照） 姉川君、おめでとうございます！

2023.2.24

We will have an on-site international workshop "Quantum Physics of Black Holes 2023" at the Yukawa Institute for Theoretical Physics on April 3 - 7. Workshop website is here. We will have lots of exciting speakers from abroad including Prof. Dan Kabat, my friend, and my former PhD mentor at Columbia. There are also short talk slots. If you wish to give a short talk, please apply for it at the time of registration. In case of applying for a short talk, the deadline for registration is March 20th. We are looking forward to your participation at this very interesting workshop!

2023.1.16

Our new postdoc, Dr. Nicolò Zenoni from Italy arrived in Japan! He recently received a double PhD from UCSC, Brescia and Leuven U. Let's work hard and write an excellent paper together!

2022.12.26

We had the 2nd ExU Annual meeting during Dec.26-28 @Kobe Convention Center. On the 26th, I gave a group activity report on this year and also gave a brief description of our hourglass prescription of entanglement entropy. In addition to meeting my old friends, I met several new faces who showed interest in our work and I enjoyed this meeting a lot. Some of the contents are available online. Program are available here.

2022.11.2

Our new postdoc, Dr. Sunil Kumar Sake from TIFR arrived in Japan! He is Sandip's excellent student and from this November he will start his new postdoc career. We are already collaborating and preparing a paper together. Welcome to Japan, Sunil! Let's work hard and write an excellent paper together!

2022.10.20

Our paper with Dan and Takanori about 4-d Maxwell theory entanglement entropy by hourglass prescription is accepted for PRD!

2022.10.16

This weekend, we had a the 1st black hole and quantum gravity workshop here at Osaka university Nambu hall. It was a very productive 2 days and I enjoyed it. Many experts in our field gave a talk on recent stuff, including Prof. Tezuka's (from Kyoto) recent development on SYK, Prof. Tamaoka's (from Nihon U.) half-wormholes, Prof. Sakai's (from Meiji-Gakuin) a big lecture series on Multi-boundary/multi-brane amplitudes in JT gravity, Prof. Ishibashi's (from Kindai) lecture on de Sitter entropy, and Prof. Maeda's (from Shibaura U.) lecture on singularity on extremal black holes in Anti-de Sitter space. I learned quite a lot and hopefully, this leads us to productive collaborations for future work. Certainly, we will do the next one in the future.

2022.09.12

Today I gave an online zoom seminar talk for the TIFR string theory group people. If you are interested, the video link of the talk is here. The topic is our recent hourglass prescription to calculate entanglement entropy, with Dan and Takanori. It was full of discussion and extremely productive 2 hours I spent, and I enjoyed talking to them and learned a lot today.

2022.09.06

Our paper is accepted for JHEP.

2022.08.01

Today I gave an online zoom NITEP colloquium talk for Osaka Metropolitan University Joint seminar of mathematical physics and particle physics. Lots of excellent questions and I enjoyed it a lot!

2022.07.26

Today, I visited Nagoya University and gave a seminar talk on our recent entanglement entropy calculation using the hourglass prescription. Thank you Sugishita-kun for the kind invitation.

2022.07.6

In the last 3 days, I gave a series of intensive Zoom lectures on "Introduction to AdS/CFT correspondence" for advanced grad students and postdocs. Unfortunately, the talk is in Japanese but at least Japanese students interested in this topic can enjoy it. If you are interested, the video link of the talk is here. Hope you found these useful! These lectures are parts of the Japanese school of our MEXT-KAKENHI-Grant-in-Aid for Transformative Research Areas (A) "Extreme Universe" activity.

2022.06.10

Today our new paper appears on arXiv. A long time back, we found an attractor mechanism in non-supersymmetric but extremal black holes in here. In this paper, with my great collaborator Prof Ishibashi-san (from Kindai) and Prof. Maeda-san (from Shibaura U.), we added deformation to these attractor extremal black holes and found that some of them flow to p.p. singular black holes!

2022.06.08

Today we had the 7th Extreme Universe Online Colloquium, the speaker is our friend Prof. Sandip Trivedi from TIFR. His talk is excellent so I highly recommend it. Youtube video are available. The slide is also available at the extreme universe homepage here.

2022.05.02

Our new paper appears on arXiv today. The results are cute, I will explain the details later but in short, we evaluate the entanglement entropy for 4-dimensional Maxwell theory in a spherical entangling surface by using our hourglass prescription and obtained the right answer − 16/90. This is quite nontrivial to me. Thank you very much, Dan and Takanori for the great collaborations.

2022.4.02

Here in 「Recent topics/最近の話題」, I will update latest news./「Recent topics/最近の話題」では主に、最近arXivに出した私達の論文の軽い背景説明や、参加した研究会等の話題や、また研究関連のニュース記事などを定期的に書いていこうと思います。

2024.7.26

My affiliation changes after August 2024 from Osaka University to National Tsing Hua University in Taiwan and Yukawa Institute, Kyoto University/2024年の8月から私の所属が大阪大学から変わります。

2022.4.1

My brand new homepage was released/ホームページを公開しました。