2019年度セミナー
今年度前期は水曜日午後１時３０分からセミナーを行います。 このセミナーの他にも、毎週木曜日１５時から本郷理学部４号館３階１３２０号室にて行われる統計力学セミナーにも参加しています。
| 日程 Date |
時間 Time |
講演者 Speaker |
演題・Abstract Title and Abstract |
| 05月08日(水) Wed, May 8 |
13:30 |
吉田恒也さん（筑波大） Dr. Tsuneya Yoshida (Tsukuba Univ.) |
Symmetry-protection of non-Hermitian degeneracies for correlated systems Abstract |
| 05月15日(水) Wed, May 15 |
13:30 |
井戸康太さん（東大物性研） Dr. Kota Ido (ISSP, Univ. Tokyo) |
Variational Monte Carlo method for electron dynamics Abstract |
| 05月22日(水) Wed, May 22 |
13:30 |
小澤知己さん（理研） Dr. Tomoki Ozawa (RIKEN) |
Quantum geometric tensor in ultracold gases and other synthetic quantum systems Abstract |
| 05月29日(水) Wed, May 29 |
13:30 |
山本薫さん（物材機構） Dr. Kaoru Yamamoto (NIMS) |
First-principles calculation of the Seebeck coefficient for Fe/MgO/Fe magnetic tunneling junction Abstract |
| 06月05日(水) Wed, Jun 05 |
13:30 |
桑原知剛さん（理研） Dr. Tomotaka Kuwahara (RIKEN) |
Approximate quantum Markov network at finite temperatures Abstract |
| 06月12日(水) Wed, Jun 12 |
13:30 |
川本達郎さん（産総研） Dr. Tatsuro Kawamoto (AIST) |
An algorithmic detectability limit of community detection in graphs Abstract |
| 06月19日(水) Wed, Jun 19 |
13:30 |
Fabio Bagarello さん（パレルモ大） Dr. Fabio Bagarello (Univ. Palermo) |
Recent results on non self-adjoint Hamiltonians Abstract |
| 07月10日(水) Wed, Jul 10 |
13:30 |
中村統太さん（芝浦工大） Dr. Tota Nakamura (SIT) |
Machine learning as an improved estimator for magnetization curve and spin gap Abstract |
| 07月24日(水) Wed, Jul 24 |
13:30 |
Katsuya Akamatsu さん（アテネオ・デ・マニラ大） Mr. Katsuya Akamatsu (Ateneo de Manila Univ.) |
Quantum annealing on Ising spin glasses Abstract |
| 10月09日(水) Wed, Oct 09 |
13:30 |
Fabrizio Minganti さん（理研） Dr. Fabrizio Minganti (RIKEN) |
Quantum exceptional points of non-Hermitian Hamiltonians and Liouvillians Abstract |
| 10月23日(水) Wed, Oct 23 |
13:30 |
X博士 Dr. X |
Mysterious Topic Abstract |
| 10月30日(水) Wed, Oct 30 |
13:30 |
島田茂さん（産総研） Dr. Shigeru Shimada (AIST) |
A Challenge in Silicone Chemistry Abstract |
| 11月20日(水) Wed, Nov 20 |
13:30 |
Hyun-Yong Lee さん (東大物性研) Dr. Hyun-Yong Lee (ISSP, Univ. Tokyo) |
Gapless Kitaev Spin Liquid to Loop and String Gases through Tensor Networks Abstract |
| 11月27日(水) Wed, Nov 27 |
13:30 |
Yogesh Joglekar さん（IUPUI, USA） Dr. Yogesh Joglekar (IUPUI, USA) |
Complete set of conserved quantities in parity-time symmetric systems: theory, observation, and consequences Abstract |
| 12月11日(水) Wed, Dec 11 |
13:30 |
本山裕一さん（東大物性研） Dr. Yuichi Motoyama (ISSP, Univ. Tokyo) |
Analytic continuation methods from imaginary time correlation function to real frequency spectrum Abstract |
| 12月18日(水) Wed, Dec 18 |
13:30 |
鈴木正さん（埼玉医科大学） Dr. Sei Suzuki (Saitama Medical Univ.) |
Quantum annealing in a thermal environment Abstract |
| 01月08日(水) Wed, Jan 08 |
13:30 |
土屋俊二さん（中央大学） Dr. Shunji Tsuchiya (Chuo Univ.) T/A |
Entanglement transport and thermalization in an isolated quantum spin chain Abstract |
| 01月15日(水) Wed, Jan 15 |
13:30 |
白崎良演さん（横国大） Dr. Ryoen Shirasaki (Yokohama Natl. Univ.) |
Theoretical study of Josephson-junction flux qubit using phase Hamiltonian of superconducting loops Abstract |
第０１回 No. 01
講師 Speaker: 吉田恒也さん（筑波大）Dr. Tsuneya Yoshida (Tsukuba University)日時 Date: ０５月０８日（水）午後１時３０分〜 Wed, May 8, 1:30pm
演題 Title: Symmetry-protection of non-Hermitian degeneracies for correlated systems
要旨 Abstract: In this decade, topological phases have attracted much interest. As the results of extensive analysis, a variety of topological insulators/superconductors have been reported which arise from interplay between symmetry and the topological properties.
In parallel to the above progress, non-Hermitian systems [1] have been pioneered as new platforms of topological physics [2]. Notably, the platforms of non-Hermitian topological physics extend to a wide range of systems; cold atoms out of equilibrium [2], correlated systems in equilibrium [3,4] etc. As this field has been pioneered very recently, many significant issues remain open questions. One of them is the interplay between symmetry and exceptional points which are topological non-Hermitian degeneracies.
We here address this issue by analyzing correlated systems. Our analysis discovers symmetry-protected non-Hermitian degeneracies [5]. By employing the dynamical mean-field theory, we demonstrate the emergence of symmetry-protected exceptional rings for a honeycomb Hubbard model. If time allows, we also show the emergence of symmetry-protected exceptional rings for classical systems [6], indicating the ubiquity of the symmetry-protected non-Hermitian degeneracies.
参考文献 Refernces
[1] N. Hatano and D. R. Nelson, Phys. Rev. Lett. 77 570 (1996).
[2] Z. Gong, Y. Ashida, K. Kawabata, K. Takasan, S. Higashikawa, and M. Ueda, Phys. Rev. X 8 031079 (2018).
[3] V. Kozii and L. Fu, arXiv: 1708.05841.
[4] T. Yoshida, R. Peters, and N. Kawakami, Phys. Rev. B 98 035141 (2018).
[5] T. Yoshida, R. Peters, N. Kawakami, and Y. Hatsugai, Phys. Rev. B 99 035141 (2019).
[6] T. Yoshida and Y. Hatsugai, submitted.
第０２回 No. 02
講師 Speaker: 井戸康太さん（東大物性研）Dr. Kota Ido (ISSP, University of Tokyo)日時 Date: ０５月１５日（水）午後１時３０分〜 Wed, May 15, 1:30pm
演題 Title: Variational Monte Carlo method for electron dynamics
要旨 Abstract: The variational Monte Carlo (VMC) method is a powerful method without the sign problem to perform simulations on quantum many-body systems. This method has been applied to investigate physical properties in a wide range of strongly correlated electron systems. Although most applications of the VMC method were limited to analyses of the ground states, it has been recently shown that the calculations of excited states such as nonequilibrium transient states are possible. In this talk, I present our recent work on the VMC method for correlated electron dynamics and its application to the Hubbard model. I also explain open source software mVMC, which has been recently developed for users to easily perform VMC simulations.
第０３回 No. 03
講師 Speaker: 小澤知己さん（理研）Dr. Tomoki Ozawa (RIKEN)日時 Date: ０５月２２日（水）午後１時３０分〜 Wed, May 22, 1:30pm
演題 Title: Quantum geometric tensor in ultracold gases and other synthetic quantum systems
要旨 Abstract: Topological and geometrical property of bands have attracted great attention during the past decade due to the development of the study of topological phases of matter in solid-state electron systems. Probably the most well-studied geometrical property of bands is the Berry curvature, integral of which gives rise to the topological Chern number. A less well known geometrical property is the quantum metric, or the Fubini-Study metric, which provides a metric structure in the Brillouin zone. Both Berry curvature and quantum metric are defined through momentum-space derivative of Bloch states, and are both gauge invariant. In fact, we can uniformly describe both concepts in terms of the quantum geometric tensor, real part of which is the quantum metric and imaginary part is the Berry curvature. In this talk, I explain intuitive meaning of the quantum metric, and discuss some physical consequences. I will also discuss recent experimental measurements of the quantum metric in ultracold atomic gases and diamond NV-centers, where the quantum metric was extracted through observation of excitation rates upon periodic modulation to the system.
第０４回 No. 04
講師 Speaker: 山本薫さん（物材機構）Dr. Kaoru Yamamoto (NIMS)日時 Date: ０５月２９日（水）午後１時３０分〜 Wed, May 29, 1:30pm
演題 Title: First-principles calculation of the Seebeck coefficient for Fe/MgO/Fe magnetic tunneling junction
要旨 Abstract: Recent progress of spin caloritronics enables us to manipulate heat and spin currents. One of the emerging interesting phenomena in spin caloritronics is the analogue of the classical Seebeck effect, such as the magneto-Seebeck effect in magnetic tunneling junctions (MTJs) [1], which is caused by spin-dependent charge current combined with heat current in parallel and anti-parallel magnetization configurations. However, understanding of the magneto-Seebeck effect in MTJs from the property of the material has not been developed so much, although it has been measured and calculated in previous studies [1,2].
In the present work, we calculate the Seebeck coefficients of Fe(7ML)/MgO(nML)/Fe(7ML) MTJ using the first-principles density functional method. The electronic transport coefficients of the MTJs are calculated from the Landauer formula. We find that the interface resonanant tunneling around the Fermi level [3] can enhance the Seebeck effect and that the effect of the resonancant tunneling depends on the in-plane lattice constant of the MTJ and the number of MgO layers [4]. Our results will be important for designing MTJs with high Seebeck coefficient.
参考文献 References
[1] M. Walter et al., Nat. Mater. 10, 742 (2011).
[2] T. Kuschel et al., J. Phys. D: Appl. Phys. 52, 133001 (2019).
[3] K. D. Belashchenko et al., Phys. Rev. B 72, 140404(R) (2005).
[4] K. Yamamoto, K. Masuda, K. Uchida, and Y. Miura, in preparation.
第０５回 No. 05
講師 Speaker: 桑原知剛さん（理研）Dr. Tomotaka Kuwahara (RIKEN)日時 Date: ０６月０５日（水）午後１時３０分〜 Wed, Jun 05, 1:30pm
演題 Title: Approximate quantum Markov network at finite temperatures
要旨 Abstract: In recent years, the Gibbs sampling on quantum computer attracts more and more attentions due to the application to exponential quantum speed up of the semidefinite programming problem [1] and the appearance of machine learning using a quantum Boltzmann machine [2]. Here, the quantum Gibbs states are described by e^{-βH}/Z (β: inverse temperature) for the system Hamiltonian H. As methods of quantum Gibbs sampling, Quantum metropolis sampling algorithm [3] and Davies Gibbs sampling algorithm [4] have been well-known. These algorithms heuristically works well, but the precision analyses are generally extremely difficult and the convergence is often exponentially slower with respect to the system size (eg, spin glass system). Our motivation in this research is to clarify under what conditions the quantum Gibbs sampling is implemented efficiently.
For the purpose, we will first introduce a method to utilize the quantum Markov property. When the system is decomposed into A, B, and C subsystems, we call that a quantum state is approximately Markov if the conditional mutual information I(A,C|B) between A and C via B exponentially decays with respect to the distance between A and C. If the Gibbs state is given by the approximate Markov network, we know that the quantum sampling can be efficiently implemented by a small depth local quantum circuits [5].
In this talk, I will show that such a quantum Markov property always hold for quantum Gibbs states above a certain threshold temperature. In addition to the efficient quantum Gibbs sampling, I will also explain several implications of the quantum Markov property: the strong versions of the area law, the clustering theorem, and existence of the topological entanglement entropy. This is a joint work with Kohtaro Kato and Fernando Brandão in Caltech IQIM.
参考文献 References
[1] F. G. S. L. Brandão and K. M. Svore, IEEE 58th Annual Symposium on Foundations of Computer Science (FOCS), pp. 415 (2017).
[2] M. H. Amin, et al., Phys. Rev. X 8, 021050 (2018).
[3] K. Temme, T. J. Osborne, K. G. Vollbrecht, D. Poulin, and F. Verstraete, Nature 471, 87 (2011).
[4] M. J. Kastoryano and F. G. S. L. Brandão, Commun. Math. Phys., 344, 915 (2016).
[5] F. G. S. L. Brandão and M. J. Kastoryano, Commun. Math. Phys., 365, 1 (2019).
第０６回 No. 06
講師 Speaker: 川本達郎さん（産総研）Dr. Tatsuro Kawamoto (AIST)日時 Date: ０６月１２日（水）午後１時３０分〜 Wed, Jun 12, 1:30pm
演題 Title: An algorithmic detectability limit of community detection in graphs
要旨 Abstract: Modularity maximization [1] using greedy algorithms continues to be a popular approach toward community detection in graphs, even after various better forming algorithms have been proposed. Apart from its clear mechanism and ease of implementation, this approach is persistently popular because, presumably, its risk of algorithmic failure is not well understood. In this talk [2], we provide an insight into this issue by estimating the algorithmic performance limit of the stochastic block model inference using modularity maximization. This is achieved by counting the number of metastable states under a local update rule [3]. Our results offer a quantitative insight into the level of sparsity at which a greedy algorithm typically fails.
参考文献 References
[1] M. E. J. Newman and M. Girvan, Phys. Rev. E 69, 026113 (2004).
[2] T. Kawamoto and Y. Kabashima, Phys. Rev. E 99, 010301(R) (2019).
[3] F. Tanaka and S. F. Edwards, J. Phys. F: Metall. Phys. 10, 2769 (1980).
第０７回 No. 07
講師 Speaker: Fabio Bagarello さん（パレルモ大）Dr. Fabio Bagarello (Univ. Palermo)日時 Date: ０６月１９日（水）午後１時３０分〜 Wed, Jun 19, 1:30pm
演題 Title: Recent results on non self-adjoint Hamiltonians
要旨 Abstract: We discuss some recent results on quantum systems whose dynamics is driven by certain non self-adjoint Hamiltonians. In particular, after a short introduction on modified commutation and anti-commutation relations, our plan is to discuss a finite-dimensional version of the CCR, a possible deformed version of the so-called generalized Heisenberg algebra, and a no-go result for the damped quantum harmonic oscillator. We also will discuss some results on tridiagonal non self-adjoint factorizable Hamiltonians, and on their SUSY counterparts.
第０８回 No. 08
講師 Speaker: 中村統太さん（芝浦工大）Dr. Tota Nakamura (SIT)日時 Date: ０７月１０日（水）午後１時３０分〜 Wed, Jul 10, 1:30pm
演題 Title: Machine learning as an improved estimator for magnetization curve and spin gap
要旨 Abstract: A magnetization curve is obtained by a numerical differentiation of the ground-state energy as . Since the energy data are discrete, the magnetization curve shows a step-wise behavior.
In this talk, we introduce a machine learning algorithm known as the Gaussian kernel regression (GKR) to evaluate a continuous magnetization curve out of discrete ground-state energy data. The spin gap is also obtained from the magnetic field at which the magnetization curve begins. The method is tested in the bond-alternation XY chain and the kagome antiferromagnet. We may replace the conventional numerical differentiation and the data extrapolation by this method.
参考文献 References
[1] K. Harada, Phys. Rev. E 84, 056704 (2011).
[2] T. Nakamura, Phys. Rev. E 93, 011301(R) (2016).
[3] T. Nakamura, arXiv:1902.02941.
第０９回 No. 09
講師 Speaker: Katsuya Akamatsu さん（アテネオ・デ・マニラ大）Mr. Katsuya Akamatsu (Ateneo de Manila Univ.)日時 Date: ０７月２４日（水）午後１時３０分〜 Wed, Jul 24, 1:30pm
演題 Title: Quantum annealing on Ising spin glasses
要旨 Abstract: Obtaining the ground state of an Ising spin glass is a notoriously difficult computational problem. Quantum annealing is a method that can be used to obtain configurations that are very close to the ground state. When performed in conjunction with thermal annealing, the system temperature and an applied tunneling field are gradually brought to zero.
In this talk, we discuss the effect of varying the annealing schedule applied to an Ising spin glass on the performance of the overall annealing procedure. Various approaches and modifications to the traditional quantum-thermal annealing process, including the variation of the tunneling field in a nonlinear manner relative to the temperature, the use of a local pulse process, and the spatial deformation of the temperature field, will be examined.
第１０回 No. 10
講師 Speaker: Fabrizio Minganti さん（理研）Dr. Fabrizio Minganti (RIKEN)日時 Date: １０月０９日（水）午後１時３０分〜 Wed, Oct 09, 1:30pm
演題 Title: Quantum exceptional points of non-Hermitian Hamiltonians and Liouvillians
要旨 Abstract: Exceptional points (EPs) correspond to degeneracies of open systems attracting much interest in optics, optoelectronics, plasmonics, and condensed matter physics. In the classical and semiclassical approaches, Hamiltonian EPs (HEPs) are usually defined as degeneracies of non-Hermitian Hamiltonians, such that at least two eigenfrequencies are identical and the corresponding eigenstates coalesce. HEPs result from continuous, mostly slow, non-unitary evolution without quantum jumps. Clearly, quantum jumps should be included in a fully quantum approach to make it equivalent to, e.g., the Lindblad master-equation approach. Thus, we suggest to define EPs via degeneracies of a Liouvillian superoperator (including the full Lindbladian term: LEPs), and we clarify the relations between HEPs and LEPs. We prove two main Theorems about differences and analogies between HEPs and LEPs. We explore the consequences of these theorems in several different examples.
第１１回 No. 11
講師 Speaker: X博士 Dr. X日時 Date: １０月２３日（水）午後１時３０分〜 Wed, Oct 23, 1:30pm
演題 Title: Mysterious Topic
要旨 Abstract: It's absolutely mysterious.
第１２回 No. 12
講師 Speaker: 島田茂さん（産総研）Dr. Shigeru Shimada (AIST)日時 Date: １０月３０日（水）午後１時３０分〜 Wed, Oct. 30, 1:30pm
演題 Title: A Challenge in Silicone Chemistry
要旨 Abstract: Silicone materials (oligo- and polysiloxanes) are used as irreplaceable materials in a wide range of fields owing to their excellent properties including high thermal stability, light stability and transparency, high gas permeability, electrical insulation property, and constancy of properties over a wide temperature range. For the further development of high performance siloxane materials, precise structural control of oligo- and polysiloxanes is indispensable. We have been working on the development of new catalytic processes for high performance silicone materials, which includes energy- and cost-efficient synthetic methods for silicone raw materials from SiO2 and precise structural control of oligo- and polysiloxanes. In this presentation, some of recent results will be discussed.
第１３回 No. 13
講師 Speaker: Hyun-Yong Lee さん (東大物性研) Dr. Hyun-Yong Lee (ISSP, Univ. Tokyo)日時 Date: １１月２０日（水）午後１時３０分〜 Wed, Nov. 20, 1:30pm
演題 Title: Gapless Kitaev Spin Liquid to Loop and String Gases through Tensor Networks
要旨 Abstract: Kitaev honeycomb model (KHM) is one of the rare examples of the exactly solvable quantum many-body model [1]. It hosts two distinct phases, the Z2 topologically ordered phase, which is adiabatically connected to the toric code state, and the gapless Kitaev spin liquid (KSL) phase exhibiting the low energy Majorana excitations [1]. In particular, the fact that the KSL phase can be driven into the non-Abelian topological phase by breaking the time-reversal symmetry attracts lots of interests in experimental and theoretical studies. In this talk, we try to understand the KSL from a new perspective, i.e., the loop gas (LG) and string gas (SG) states which are efficiently and compactly defined in the tensor network representation [2]. One can show in a local tensor level that the LG and SG states, which we propose, not only respect the symmetries of KSL but also satisfy the vortex-free condition [2]. Also, those are critical states characterized by 2D Ising CFT and have the Z2 gauge redundancy which allows us to create and move the vortex excitations exactly. Furthermore, accurate variational energy for the KHM is obtained with only two variational parameters [2], which ensures that our ansatze represent quantitatively as well as qualitatively the KSL. We discuss their physical properties, relation to classical statistical mechanics and topological properties in details.
参考文献 References
[1] A. Kitaev, Annals of Physics 321(2006) 2-111.
[2] H. Y. Lee, R. Kaneko, T. Okubo, and N. Kawashima, arXiv:1901.05786.
第１４回 No. 14
講師 Speaker: Yogesh Joglekar さん（IUPUI, USA）Dr. Yogesh Joglekar (IUPUI, USA)日時 Date: １１月２７日（水）午後１時３０分〜 Wed, Nov 27, 1:30pm
演題 Title: Complete set of conserved quantities in parity-time symmetric systems: theory, observation, and consequences
要旨 Abstract: Conserved quantities of a system are determined by its global, local, or accidental symmetries. They were instrumental to advances such as the prediction of neutrinos in the (inverse) beta decay process and the development of self-consistent approximate methods for isolated or thermal many-body systems. In contrast, little is known about conservation laws and their consequences in open systems. Recently, a special class of these systems, called parity-time (PT ) symmetric systems, has been intensely explored for their remarkable properties that are absent in their closed counterparts. A complete characterization and observation of conserved quantities in these systems and their consequences is still lacking. We will present a complete set of conserved observables for a broad class of PT -symmetric Hamiltonians and experimentally demonstrate their properties using single-photon linear optical circuit. By simulating the dynamics of a four-site system across a fourth-order exceptional point, we measure its four conserved quantities and demonstrate their consequences. Our results spell out non-local conservation laws in non-unitary dynamics and provide key elements that will underpin self-consistent analysis of non-Hermitian, quantum, many-body systems that are forthcoming.
参考文献 References
[1] Collaboration work with Prof. Peng Xue’s group.
第１５回 No. 15
講師 Speaker:本山裕一さん（物性研） Dr. Yuichi Motoyama (ISSP, Univ. Tokyo)日時 Date: １２月１１日（水）午後１時３０分〜 Wed, Dec. 11, 1:30pm
演題 Title: Analytic continuation methods from imaginary time correlation function to real frequency spectrum
要旨 Abstract: It is important to calculate dynamical quantities such as the spectrum function (density of states) and the dynamical structure factor by numerical methods because they reflect on the structure of low-energy excited states and they are observed by experiments, e.g., ARPES. By using the path-integral Monte Carlo method, these quantities can be calculated through the so-called "analytic continuation" (AC) method from the imaginary-time correlation functions such as the temperature Green's function. The AC method, unfortunately, is well-known as ill-posed problem; the noise (the statistical error) of the correlation function (input) is exponentially magnified in the spectrum function (output). In order to overcome this instability, several AC methods have been developed in decades. For example, the SpM method adopts the sparse modeling method for reducing the noise, so that SpM gets a strong robustness. The SpM method, unfortunately, suffers from an unphysical oscillation due to the truncation. In this talk, I will introduce our new method, the SpM-Pade method, which is based on two existing methods, the Pade method and the SpM method. I will also review other AC methods, the maximum entropy method and the deep learning method.
第１６回 No. 16
講師 Speaker:鈴木正さん（埼玉医科大学）Dr. Sei Suzuki (Saitama Medical Univ.)日時 Date: １２月１８日（水）午後１時３０分〜 Wed, Dec. 18, 1:30pm
演題 Title: Quantum annealing in a thermal environment
要旨 Abstract: Quantum annealing is a quantum computing algorithm specific to optimization problems of seeking the optimum item from many others. Since D-Wave's quantum annealing machine has been developed, quantum annealing has attracted quite a lot of attentions. Quantum annealing was originally proposed for an isolated quantum spin system. However, systems realized in quantum annealing machines interact with an environment. Hence it is important to study quantum annealing in an environment. We at first developed a numerical method to compute time evolution of a reduced density matrix for spins in a one-dimensional transverse Ising model coupled to a bosonic bath, using the time evolving block decimation (TEBD) or infinite TEBD (iTEBD) method and the quasi-adiabatic propagator path integral (QUAPI) formalism. This method enables one to compute time evolution of systems with O(10^2) spins or infinite homogeneous systems without relying on the Markov approximation. We then studied quantum annealing in the thermal environment with a finite temperature with our method. If quantum annealing is performed with infinitely long time in a thermal environment, the spin system would evolve quasi-statically. When it is done with a finite speed, the quasi-static evolution breaks at some time and the spin system is frozen there. We confirmed this and found that the freezing time and the energy after annealing obey a universal scaling law.
第１７回 No. 17
講師 Speaker:土屋俊二さん（中央大学）Dr. Shunji Tsuchiya (Chuo Univ.)日時 Date: ０１月０８日（水）午後１時３０分〜 Wed, Jan. 8, 1:30pm
演題 Title: Entanglement transport and thermalization in an isolated quantum spin chain
要旨 Abstract: Entanglement transport is considered to be essential for understanding thermalization in an isolated quantum system. In this work, we study transport of entanglement entropy (EE) in the Ising model with the next-nearest-neighbor interaction as well as the transverse and longitudinal magnetic fields. We calculate EE in a spin chain which consists of a single spin at the edge (A) and the bulk part (B). We compare time-evolution of EE for two different initial settings: the one with entanglement between A and B and the other one without it. The propagation speed of EE can be estimated by the time at which the EEs starting from the two initial conditions show deviation. We find that EE propagates ballistically with a constant velocity and that the propagation speed is enhanced when thermalization occurs. We also calculate the propagation speed of EE from mutual information between A and a subsystem in B. The velocities calculated by the two different methods agree well. We will discuss the relation between propagation of EE and thermalization.
第１８回 No. 18
講師 Speaker:白崎良演さん（横国大） Dr. Ryoen Shirasaki (Yokohama Natl. Univ.)日時 Date: ０１月１５日（水）午後１時３０分〜 Wed, Jan. 15, 1:30pm
演題 Title: Theoretical study of Josephson-junction flux qubit using phase Hamiltonian of superconducting loops
要旨 Abstract: A flux qubit is a superconducting loop including several Josephson-junctions realizing quantum superposition of superconducting states. A superconducting loop with 3 Josephson-junction (3-JJ) runs as a qubit with a single 1/2 spin, while a double superconducting loop qubit with 4 Josephson-junction (4-JJ) is a tunable qubit in which the potential profile can be varied by applied magnetic fluxes. We theoretically studied the operations of these flux qubits using a Hamiltonian of superconducting loops.