研究室メンバーによるレギュラー・セミナー、 および、他研究室・機関からのゲスト・スピーカーのセミナーを開催しています。 レギュラー・セミナーは下記要領で行います。 Access map：理学部1号館 (Faculty of Science Bldg. 1) / 理学部4号館 (Faculty of Science Bldg. 4)
夏学期 / Summer semester (April-July 2024)
<Regular seminars (review of seminal papers)>木曜日13時から理学部１号館414号室で行います(普段と場所or時間の異なる場合は赤字で示します)。
Each seminar starts from 13:00, Thursday @ #414, Faculty of Science Bldg. 1 (unless otherwise indicated).
<Schedule>
Apr. 18 Hokkyo
"The Classical Limit of Quantum Spin Systems"
E. H. Lieb, Commun.Math. Phys. 31, 327-340 (1973).
Apr. 25 (No seminar)
May. 2 Shiraishi
"Theory of Thermal Transport Coefficients"
J. M. Luttinger, Phys. Rev. 135, A1505 (1964).
"A Quantum-statistical Theory of Transport Processes"
H. Mori, J. Phys. Soc. Jpn. 11 1029 (1956).
May. 9 Li
"Exact Analysis of an Interacting Bose Gas. I. The General Solution and the Ground State"
E. H. Lieb and W. Liniger, Phys. Rev. 130, 1605 (1963).
"Exact Analysis of an Interacting Bose Gas. II. The Excitation Spectrum"
E. H. Lieb, Phys. Rev. 130, 1616 (1963).
May. 16 (No seminar)
May. 23 (No seminar)
May. 30 Sakamoto(starting from 3pm)
"Quantum cryptography: Public key distribution and coin tossing"
C.H. Bennett, G. Brassard, Proceedings of IEEE International Conference on Computers Systems and Signal Processing, Bangalore India, 175 (1984).
Jun. 6 Sugiura
"The renormalization group and the ϵ expansion"
Kenneth G. Wilson and J. Kogut, Physics Reports 12, 2, 75-199 (1974).
Jun. 13 Ishii
"'Luttinger liquid theory' of one-dimensional quantum fluids. I. Properties of the Luttinger model and their extension to the general 1D interacting spinless Fermi gas"
F. D. M. Haldane, Journal of Physics C: Solid State Physics, 14(19), 2585 (1981).
Jun. 20 Ogawa (B4 student)
"The Uncertainty Relation Between Energy and Time in Non-relativistic Quantum Mechanics"
L. Mandelstam and Ig. Tamm, J. Phys. USSR 9, 249-254 (1945).
"The maximum speed of dynamical evolution"
Margolus Norman and Lev B. Levitin, Physica D: Nonlinear Phenomena 120.1-2 (1998): 188-195.
"New form of the time-energy uncertainty relation"
Eric A. Gislason, Nora H. Sabelli, and John W. Wood, Phys. Rev. A 31, 2078 (1985).
Jun. 27 (No seminar)
Jul. 4 Kandabashi (B4 student)
"Nonequilibrium Equality for Free Energy Differences"
C. Jarzynski, Phys. Rev. Lett. 78(14) 2690 (1997).
Jul. 11 Oyaizu (@233 → changed to 913)
"Classical Time Crystals"
Alfred Shapere and Frank Wilczek, Phys. Rev. Lett. 109, 160402 (2012).
"Quantum Time Crystals"
Frank Wilczek, Phys. Rev. Lett. 109, 160401 (2012).
冬学期 / Winter semester (October 2024-January 2025)
木曜日13時から理学部１号館913号室で行います(普段と場所or時間の異なる場合は赤字で示します)。Each seminar starts from 13:00, Thursday @ #913, Faculty of Science Bldg. 1 (unless otherwise indicated).
<Schedule>
10/3 (No seminar)
10/10 Kai Li/Hongchao Li (@414)
10/17 Sakamoto
10/24 Hokkyo
10/31 Ishii
11/7 (No seminar)
11/14 (No seminar)
11/21 (No seminar)
11/28 Nakagawa
12/5 (No seminar)
12/12 Shiraishi
12/19 Sugiura
12/26 (No seminar)
1/9 Oyaizu (@207)
1/16 Nohara / Yamada (@TBA)
| speaker | Kai Li |
| title | Probing non-Hermitian eigenenergies |
| abstract |
While non-Hermitian Hamiltonians have been experimentally realized in cold atom systems, it remains an outstanding open question of how to experimentally measure their complex energy spectra in momentum space for a realistic system with boundaries. The existence of non-Hermitian skin effects may make the question even more difficult to address given the fact that energy spectra for a system with open boundaries are dramatically different from those in momentum space; the fact may even lead to the notion that momentum-space band structures are not experimentally accessible for a system with open boundaries. In this study [1], we generalize the widely used radio-frequency spectroscopy to measure both real and imaginary parts of complex energy spectra of a non-Hermitian quantum system for either bosonic or fermionic atoms. By weakly coupling the energy levels of a non-Hermitian system to auxiliary energy levels, we theoretically derive a formula showing that the decay of atoms on the auxiliary energy levels reflects the real and imaginary parts of energy spectra in momentum space. We prove that measurement outcomes are independent of boundary conditions in the thermodynamic limit, providing strong evidence that the energy spectrum in momentum space is experimentally measurable. We also discuss whether the spectrum under open boundary conditions can be measured when skin effects exist and how the interaction in the non-Hermitian system affects the measurement results.
[1] K. Li and Y. Xu, Phys. Rev. Lett. 129, 093001 (2022). |
| speaker | Hongchao Li |
| title | Dissipative Superfluidity in a Molecular Bose-Einstein Condensate |
| abstract |
Quantum gases of dipolar molecules, which serve as a platform to realize clean and controllable long-range interacting systems, have received considerable attention in the fields of many-body physics and quantum simulation. However, heteronuclear molecules inevitably suffer the two-body loss due to chemical reactions, which is particularly serious for bosonic molecules. Recently, with the development of the microwave shielding
the first experimental realization of a BEC of heteronuclear molecules has been reported. Thus, it is of fundamental interest to understand whether or not superfluidity exists under two-body loss in such BECs, since dissipation may deteriorate the phase coherence of a superfluid. In this study [1], we develop superfluid transport theory for a dissipative BEC to show that a weak uniform two-body loss can induce phase rigidity, leading to superfluid transport of bosons even without repulsive interparticle interactions. We also show a generalized f-sum rule for a dissipative superfluid as a consequence of weak U(1) symmetry. Finally, we demonstrate that dissipation enhances the stability of a molecular BEC with dipolar interactions.
[1]: H. Li, X. Yu, M. Nakagawa, M. Ueda, arXiv: 2406.08868. |
| speaker | Yuki Sakamoto |
| title | Phase diagrams in evolutionary games on graphs |
| abstract |
Game theory has been applied to various fields such as politics, psychology and biology.
In particular, some game-theoretical models are considered to enhance cooperation even in social viscosity in a large population.
In recent years, several mechanisms have been found to construct sustainable cooperation by giving players mutual advantages and keeping them from undesirable risk avoidance.
One mechanism is a network structure.
We consider a multiplayer prisoner's dilemma game on a graphs, where each player is placed at a node and interacts with his/her neighbors connected by edges.
Players change their strategis in time based on the pairwise-Fermi update rule.
We obtain phase diagrapms with respect to the fraction of cooperators as a function of the strength of natural selection and a possible benefit in the payoff matrix.
In the phase diagram, there appears a diamond-shaped mixed region where cooperators and defectors coexist.
We discuss how the phase diagram depends on the graph structure and the update rule.
[1]: Y. Sakamoto and M. Ueda, Phys. Rev. E 110, 034110 (2024). |
| speaker | Akihiro Hokkyo |
| title | Wigner-Araki-Yanase Theorem Beyond Unitary Symmetry |
| abstract | The Wigner-Araki-Yanase theorem (WAY theorem) asserts that when conserved quantities exist in indirect measurements, it is impossible to accurately measure observables that are not commute with conserved quantities. Conserved quantities are equivalent to continuous unitary symmetries and the theorem can be generalised to the case of discrete unitary symmetries. Therefore, we can say that the WAY theorem represents a restriction that the symmetry of the measurement model imposes on the implemented measurement. However, the WAY theorem in non-unitary symmetries, such as time-reversal symmetries, has not been studied. In this presentation, I discuss a possible extensions of the WAY theorem to non-unitary symmetries. In particular, I show that naive extensions of the WAY theorem do not hold for time-reversal symmetries and that there are certain restrictions due to time-reversal symmetry, when we consider the measurement process rather than just the probability distribution of the measurement. |
| speaker | Takanao Ishii |
| title | Tunable Quantum i.i.d. Steady States in Open Quantum Systems |
| abstract |
In closed quantum many-body systems, the quantum phase transition is a dramatic change of the ground state as a function of external parameters. The corresponding phenomenon in open quantum many-body systems is dissipative quantum phase transition, which is the dramatic change of the steady state as a function of external parameters. As the ground state is important in closed quantum many-body systems, calculating the steady state is essential to studying open quantum many-body systems. Time evolution by Hamiltonian creates spatial correlations, while dissipation reduces spatial correlations. In most of the systems, these two effects compete against each other, and the spatial correlation remains in a steady state. However, in some special cases, the steady state becomes quantum i.i.d. state, which means the absence of spatial correlation. In this seminar, the sufficient condition to have quantum i.i.d. steady state, and the necessary condition to have tunable quantum i.i.d. steady state are discussed. Also, the classification of the system with quantum i.i.d. steady state will be mentioned. Furthermore, calculation of a time correlation function is to be discussed. The class of system that has quantum i.i.d. steady state would be a great toy model to study nonequilibrium statistical mechanics of quantum many-body systems since the analytical calculations can be easily performed. |