|title||Topological pump of Weyl fermion and Floquet chiral magnetic effect|
The periodicity of Brillouin zones imposes strong topological constraints on realizable band structures. For example, in a three-dimensional lattice system, left- and right-handed Weyl fermions must appear in pairs and hence it is impossible to realize a single Weyl fermion (Nielsen-Ninomiya theorem ). However, in periodically driven systems, topology of quasi-energy band structures of Floquet states is richer than static systems due to the time-periodic structure of Hamiltonians . |
In this talk, we present a model which realizes a single Weyl fermion on a periodically driven three-dimensional lattice . Our model is a three-dimensional analog of the topological Thouless pump. Reflecting the spin-momentum-locking nature of a Weyl fermion, a spin-polarized wave packet moves parallel to its spin direction in this pump. Furthermore, we show that, under this pump with a magnetic field, a wave packet moves parallel to the applied magnetic field, which is a Floquet analog of the chiral magnetic effect.
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 T. Kitagawa, E. Berg, M. Rudner, and E. Demler, Phys. Rev. B 82, 235114 (2010).
 S. Higashikawa, M. Nakagawa, and M. Ueda, in preparation.
|speaker||Shuhei M. Yoshida|
|title||Universality of an Impurity in a Bose-Einstein Condensate|
Physicists aspire to find universality, where comlex phenomena are described by a small number of parameters. The unitary Fermi gas is a prime example, whose ground-state thermodynamics is completely determined by its density and the Bertsch parameter. The strongly correlated Bose gases, on the other hand, always have the three-body parameter, which emerges due to the Efimov effect and characterizes the three-body physics. One may ask, is the three-body parameter enough, or do we need other details to characterize a Bose gas? We address this question for a Bose polaron, which is a single impurity immersed in a Bose gas. We calculate the ground-state energy of a Bose polaron using two different models of interactions. By comparing their results and also the results of another model in the previous study , we show that the ground-state energy is a model-independent function of the three-body parameter if the background Bose gas is not too dense.|
 S. M. Yoshida, S. Endo, J. Levinsen, and M. M. Parish, arXiv:1710.02968.
 L. A. P. Ardila, and S. Giorgini, Phys. Rev. A 92, 033612 (2015).
|title||Evaluating excess work in thermodynamic control|
One of difficulties in analyzing nonequilibrium processes is that physical quantities at a certain time generally depend on the history of the system. In particular, we consider excess work in nonequilibrium processes where the system is controlled by a time-dependent Hamiltonian or external potential. When the control is slow, the average excess work can be approximately evaluated based on the thermodynamic metric on the control parameter space, in which the work performed into the system at a certain time depends only on the velocity of the control parameter at that time.|
In this seminar, we extend this thermodynamic metric formalism in two directions. First, we systematically expand the average excess work in terms of the slowness of the control in a phenomenological manner, where the most leading term coincides with the thermodynamic metric. Second, we formulate a method to determine higher order cumulants in overdamped Langevin systems.
|title||Atypicality of most few-body observables|
Understanding how isolated quantum systems thermalize has recently gathered renewed interest among theorists, thanks to the experimental realizations of such systems.
If the eigenstate thermalization hypothesis (ETH) holds true, the microcanonical ensemble is justified as a steady-state ensemble .
The ETH states that diagonal matrix elements of an observable for the energy eigenstates are almost the same within a small energy shell.
One possible explanations of the ETH has been the typicality argument, which predicts the exponentially small variation of diagonal matrix elements with the size of the system .
In this seminar, however, we show that the argument does not apply to most few-body observables for few-body Hamiltonians unless the width of the energy shell decreases exponentially with increasing the size of the system .
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 R. Hamazaki and M. Ueda, arXiv:1708.04772 (2017).
|title||Transient fractality as a mechanism of emergent irreversibility in chaotic Hamiltonian dynamics|
The Loschmidt paradox claims that one cannot obtain macroscopic irreversible behavior from microscopic reversible dynamics since there exists one-to-one correspondence between a trajectory and its time-reversed one. Boltzmann refuted this claim by arguing that trajectories with a positive entropy production are realized much more frequently than those with a negative entropy production despite this one-to-one correspondence. His idea was later elucidated in chaotic systems ruled by dissipative reversible equations of motion . In their discussion, fractality in phase space plays a key role to account for emergent irreversibility. In this talk, we address the question of whether this picture applies to chaotic systems with conservative reversible equations of motion. Although the Liouville theorem prohibits fractality in the long time limit, we find that a fractal structure emerges in an intermediate timescale in a dynamical billiard. This transient fractality can be quantitatively evaluated by means of the time-reversal test and the Rényi divergence. Moreover, this emergent irreversibility has an intimate relation to a singular class of irreversibility called absolute irreversibility in the context of the fluctuation theorem. We suggest that the fractality is a universal mechanism to generate irreversibility from reversible equations of motion.
 B. L. Holian, W. G. Hoover and H. A. Posch, Phys. Rev. Lett. 59, 10 (1987).
|title||Parity-time-symmetry-induced edge modes in the Majorana chain|
Non-Hermitian systems with parity-time (PT) symmetry have attracted growing interest over the past two decades . Such systems exhibit unconventional spontaneous symmetry breaking that has no analogue in equilibrium systems, and recent experimental realizations [2-4] in classical systems have sparked diverse studies of phenomena unique to PT-symmetric systems. In the quantum regime, various aspects of PT-symmetric systems have been studied, such as critical phenomena  and quantum information . However, much remains to be explored concerning the interplay between PT symmetry and topology. In this talk, we study a one-dimensional topological superconductor with PT symmetry . We discover that PT symmetry breaking occurs as the emergence of new edge modes. We also find that anomalous current localized at edges is generated, which is induced by the combination of PT symmetry and topology.
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|title||Collective modes of vortex lattices in two-component Bose-Einstein condensates|
A synthetic magnetic field can be induced in neutral atoms by rotating the gas or by optically dressing atoms. For scalar Bose-Einstein condensates (BEC), a synthetic magnetic field creates quantized vortices, which form Abrikosov’s triangular lattice owing to their mutual repulsion. In the case of rotating two-component BECs, mutually parallel magnetic fields are induced in the two components, and give rise to a variety of vortex lattice structures which are controlled by the ratio of the intercomponent interaction g↑↓to the intracomponent one g [1,2]. It can be shown within the Gross-Pitaevskii mean-field theory that two component BECs in antiparallel fields, which can be created by an optical dressing technique, exhibit exactly the same vortex lattice phase diagram . |
We study collective modes of vortex lattices in two-component BECs by utilizing Bogoliubov theory and an effective theory. We numerically obtain excitation bands for each vortex lattice phase, and find the emergence of two modes with linear and quadratic dispersion relations at low energies for all phases in both the cases of parallel and antiparallel fields. We also derive dispersion relations for both the cases by using effective theory, and checked the consistency of our numerical results with the effective theory prediction.
 E. J. Mueller and T.-L. Ho, Phys. Rev. Lett 88, 180403 (2002).
 K. Kasamatsu, M. Tsubota, and M. Ueda, Phys. Rev. Lett. 91, 150406 (2003).
 S. Furukawa and M. Ueda, Phys. Rev. A 90, 033602 (2014).
|title||Isolated coarsening dynamics in a one-dimensional antiferromagnetic spinor Bose gas|
Coarsening dynamics is a ubiquitous relaxation phenomenon appearing after a sudden quench of system’s parameters across a phase transition point. Originally, this kind of relaxation has been intensively studied in various open dissipative systems such as a metal alloy and a binary liquid, and is found to be classified to some universality classes . Recently, we study a one-dimensional (1D) coarsening dynamics in an isolated ferromagnetic spinor Bose gas, analytically and numerically discovering a new universality class unique to isolated systems . Then, this study poses one question: “Is this the only universality class unique to isolated systems?”. In this talk, we discuss this issue, focusing on a 1D coarsening dynamics in an antiferromagnetic spinor Bose gas. We will show numerical results based on the truncated Wigner approximation.|
 A. J. Bray, Adv. Phys. 43, 357 (1994).
 K. Fujimoto, R. Hamazaki, and M. Ueda, arXiv:1707.03615.
|title||Discrete time-crystalline order in cavity and circuit QED systems|
Discrete time crystals are a recently proposed [1-3] and experimentally observed [4,5] dynamical phase of out-of-equilibrium Floquet systems, where the stroboscopic evolution of a local observable repeats itself at an integer multiple of the driving period. We address this issue in a driven-dissipative setup, focusing on the modulated open Dicke model , which can be implemented by cavity  and circuit QED systems . In the thermodynamic limit, we employ semiclassical approaches and find unexpectedly rich dynamical phases in addition to the discrete time-crystalline order. In a deep quantum regime with few qubits, we find clear signatures of a transient discrete time-crystalline behavior, which is absent in the isolated counterpart. We establish a general phenomenology of dissipative discrete time crystals by generalizing the celebrated Landau theory of phase transitions to Floquet open systems.|
 V. Khemani, A. Lazarides, R. Moessner, and S. L. Sondhi, Phys. Rev. Lett. 116, 250401 (2016).
 D. V. Else, B. Bauer, and C. Nayak, Phys. Rev. Lett. 117, 090402 (2016).
 N. Y. Yao, A. C. Potter, I.-D. Potirniche, and A. Vishwanath, Phys. Rev. Lett. 118, 030401 (2017).
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 Z. Gong, R. Hamazaki, and M. Ueda, arXiv:1708.01472.
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 F. Yoshihara, T. Fuse, S. Ashhab, K. Kakuyanagi, S. Saito, and K. Semba, Nat. Phys. 13, 39 (2017).
|title||Magnetic properties of volborthite determined by a coupled-trimer model|
The natural mineral volborthite hosts layers of spin-1/2 moments forming a kagome lattice. While this material was initially considered as a candidate of a spin-1/2 kagome antiferromagnet, it exhibits rich magnetic behavior which is in many respects distinct from the known features of a kagome antiferromagnet. In particular, recent single-crystal experiments have revealed a wide 1/3 magnetization plateau starting at H = 26 T, an incommensurate spin-density-wave phase below H = 23 T, and the novel ``N'' phase inbetween them.|
To explain these rich field-induced phenomena, we have performed microscopic modeling of volborthite by means of density functional theory (DFT) with the single-crystal structural data as a starting point. Using DFT+U, we find four leading magnetic exchanges: antiferromagnetic J and J2, as well as ferromagnetic J ' and J1 with a remarkable hierarchy J > |J1| > J2, |J '|. Due to the dominance of J, the magnetic planes break up into magnetic trimers. The 1/3-plateau state can be naturally interpreted as a product of polarized trimers, and a wide plateau extending to H = 225 T is predicted. Furthermore, we derive an effective pseudospin-1/2 model by restricting ourselves to the lowest-energy doublet on each trimer and treating the inter-trimer couplings perturbatively. This model shows a tendency towards condensation of magnon bound states preceding the plateau, providing a scenario for the observed ``N'' phase.
We are currently analyzing the effects of Dzyaloshinskii-Moriya interactions, which may be responsible for a low-field magnetic phase for H < 4 T and thermal Hall effect observed in recent experiments. The seminar will include a brief progress report in this direction.
Reference: O. Janson, S. Furukawa, T. Momoi, P. Sindzingre, J. Richter, and K. Held, Phys. Rev. Lett. 117, 037206 (2016).
|title||SU(3) Quantized Vortices in a Spin-1 Bose-Einstein Condensate|
|abstract||In a superfluid with internal degrees of freedom such as the A phase of a superfluid helium-3 (3He-A), the superfluid current can be characterized by the d vector as well as the U(1) phase. Its circulation is quantized when it encloses a certain type of vortices, for instance, the Mermin-Ho vortex, the Anderson-Toulouse vortex. A spin-1 BEC is analogous to the 3He-A, when the spin vector is fully-polarized; however, it exhibits richer physics of quantized vortices than the 3He-A, such as the half-quantized vortex and the polar core vortex. The half-quantized vortex can be created in an unpolarized BEC, and in the polar core vortex, the magnitude of the spin vector is varied as well as its orienation. To systematically treat all of these quantized vortices in a spin-1 BEC, we generalize the Mermin-Ho relation in terms of the eight generators and the structure factors of the su(3) algebra. The obtained equality implies the contribution to the ferromagnetic Mermin-Ho relation from each generator and a new vortex that belongs to an su(2) subalgebra of the su(3) algebra. We also illustrate the textures of the new vortices.|
|title||Dynamical aspects of a two-component Fermi gas|
|abstract||Two-component Fermi gas realized in ultracold atoms offers an optimal playground to examine a strongly interacting quantum system in a detailed manner. While thermodynamics properties of such a gas are gradually caught due to experimental and theoretical progresses, little is known on its dynamical properties. In this seminar, I will introduce our attempts to understand dynamic aspects of the two-component Fermi gas via mesoscopic transport and polaron.|
|title||Solving quantum impurity problems in and out of equilibrium with variational method|
Our-of-equilibrium phenomena in quantum impurity systems have recently come under active investigations in both experiments and theory. Following the original ideas by Tomonaga and Lee, Low and Pines, we develop a general theoretical approach to solve in- and out-of-equilibrium problems of quantum spin-impurity systems . Employing the discrete symmetry hidden in spin-impurity Hamiltonians, we introduce a new canonical transformation that can completely decouple the impurity from bath degrees of freedom. We combine it with Gaussian states to encode the impurity-bath entanglement efficiently. We benchmark our theory by comparing it with MPS results and testing universal scaling and transport phenomena in Kondo models. Some of the results are challenging to obtain in other theoretical approaches and can be tested using nanodevices and ultracold atoms .|
 Y. Ashida, T. Shi, M.-C. Banuls, J. I. Cirac, and E. Demler (in prep).
 M. Kanasz-Nagy, Y. Ashida, T. Shi, C. P. Moca, T. N. Ikeda, S. Fooling, I. Bloch, J. I. Cirac, G. Zarand, and E. Demler (in prep).
|title||OTO fluctuation-dissipation theorem|
|abstract||An out-of-time-ordered correlator (OTOC) is a multi-point correlation function such as ＜A(t)B(t’)A(t)B(t’)＞ that does not obey the usual time-ordering rule. The OTOC has recently attracted growing attention as a measure to characterize chaotic behavior in quantum many-body systems. Here we prove that a generalized fluctuation-dissipation theorem holds for a certain class of OTOCs. The theorem shows a universal relation between chaotic behavior in quantum systems and a nonlinear response function that involves a time-reversed process. We also discuss an extension of the theorem to higher-order OTOCs as well as in a different form of regularization.|
|title||Quantum metrology in a continuous system|
In general, we can reduce error by increasing the number of samples, but the rate at which error decrease varies with the method.
Quantum metrology has been used to exploit this kind of error rate in estimating quantum-dynamical parameters. It is known that, in estimating finite number of parameters, the estimation error is in proportion to 1/√N or 1/N, depending on whether the input is restricted to separable states or not. |
The estimation becomes difficult when the parameter continuously varies over time or space, where different error rates are obtained. Such rates are previously obtained by Bayesean analysis, in estimating a signal with correlated Gaussian noise. The error rate depends on high-frequency behavior of the power spectrum.
In this seminar, we discuss our recent study on the estimation of function, which yields the similar but essentially different error rates. The analysis we conduct is deterministic, that is, no prior probability needs to be imposed. Instead of the power spectrum, the error rates depends on the differentiability of the function. Furthermore, for the functions with bounded first derivatives, estimation procedure in position and wavenumber space is equally efficient, exhibiting the quantum version of the Nyquist's sampling theorem.
|title||Non-Hermitian Kondo effect in ultracold atoms|
The Kondo effect is one of the most important phenomena in strongly correlated many-body systems. It consists of a localized impurity spin and a surrounding fermion cloud which antiferromagnetically couples with the impurity, thereby realizing an emergent many-body bound state called the Kondo singlet. Motivated by the recent progress in cold-atom experiments , which have realized the Kondo Hamiltonian using ultracold Yb atoms, we extend the paradigm of the Kondo effect towards open quantum systems. We consider the Kondo Hamiltonian by taking into account the effect of inelastic scattering with the impurity spin, which gives rise to a non-Hermitian term in the exchange interaction. Using the renormalization group (RG) calculation, we find that the Kondo effect shows an anomalous RG flow in the non-Hermitian case, accompanied by a new energy scale unique to the dissipative system. We confirm our prediction of the RG flow using an exact solution based on a generalized Bethe ansatz.|
 L. Riegger, N. D. Oppong, M. Höfer, D. R. Fernandes, I. Bloch, and S. Fölling, arXiv:1708.03810.
|title||Possible realization of a Thouless pump in the Jaynes-Cummings model|
In 1983, Thouless formulated adiabatic charge pumps in one-dimensional periodically driven quantum systems with periodic-boundary conditions . He found that the particle transfer after one period is quantized as the Chern number. This can be considered as a momentum-time analog of the integer quantum Hall. In 2016, about 35 years after Thouless’s original paper, such a Thouless pump was realized in ultra-cold atom systems [2,3], and has attracted much recent interest in the context of Floquet systems . |
Previous proposals and realizations of the Thouless pump mostly exploit the real-space degrees of freedom. Here I will talk about an ongoing study on a Thouless pump in the Fock space based on the driven Jaynes-Cummings model, which can readily be implemented with cavity and circuit QED systems. As a by-product, I will discuss about a new method for generating Fock states.
 D. J. Thouless, Phys. Rev. B 27, 6083 (1983).
 S. Nakajima, T. Tomita, S. Taie, T. Ichinose, H. Ozawa, L. Wang, M. Troyer, and Y. Takahashi, Nat. Phys 12, 296 (2016).
 M. Lohse, C. Schweizer, O. Zilberberg, M. Aidelsburger, and I. Bloch, Nat. Phys 12, 350 (2016).
 T. Kitagawa, E. Berg, M. Rudner, and E. Demler, Phys. Rev. B 82, 235114 (2010).
|title||Introduction to coherence resource theory|
|abstract||Recently, to quantify coherence, the theory of coherence as a resource was proposed in parallel with the existing entanglement resource theory and asymmetry resource theory. It uses an informational and axiomatic approach, featuring simple mathematical representation and properties, but controversial physical justification, and therefore has been discussed by many authors who again proposed different possible definitions and measures. Here, I introduce the basic framework of a resource theory, the current most widely used conventions, basic axioms and their corresponding implications, the mathematical results, while roughly going through other conventions. Entanglement and translational coherence resources are also introduced for comparison, as they are closely related to the coherence resource theory both physically and mathematically. Criticism are examined, and in order to justify the unique stance of the coherence resource theory apart from others, applicable physical situations are discussed, especially for interferometer experiments and information processing, showing the physical significance for the theory.|
|speaker||Prof. Adolfo del Campo (UMass Boston)|
|title||Engineering Quantum Thermal Machines|
Quantum thermodynamics has emerged as an interdisciplinary research field in quantum science and technology with widespread applications. Yet, the identification of scenarios characterized by quantum supremacy - a performance without match in the classical world - remains challenging. In this talk I shall review recent advances in the engineering and optimization of quantum thermal machines. I will show that nonadiabatic many-particle effects can give rise to quantum supremacy in finite-time thermodynamics . |
Tailoring such nonadiabatic effects by making use of shortcuts to adiabaticity, quantum heat engines can be operated at maximum efficiency and arbitrarily high output power . A thermodynamic cost of these shortcuts will be elucidated by analyzing the full work distribution function and introducing a novel kind of work-energy uncertainty relation . I shall close by discussing the identification of scenarios with a quantum-enhanced performance in thermal machines run over many cycles .
 J. Jaramillo, M. Beau, and A. del Campo, New J. Phys. 18, 075019 (2016).
 M. Beau, J. Jaramillo, and A. del Campo, Entropy 18, 168 (2016).
 K. Funo, J.-N. Zhang, C. Chatou, K. Kim, M. Ueda, and A. del Campo, Phys. Rev. Lett. 118, 100602 (2017).
 G. Watanabe, B. P. Venkatesh, P. Talkner, and A. del Campo, Phys. Rev. Lett. 118, 050601 (2017).
|speaker||Prof. Haruki Watanabe (UTokyo)|
 H. Watanabe, H. C. Po, A. Vishwanath, and M. P. Zaletel, Proc. Natl. Acad. Sci. U.S.A. 112, 14551 (2015); H. C. Po, H. Watanabe, C.-M. Jian, and M. P. Zaletel, arXiv:1703.06882. 日本物理学会誌 2017年1月号に解説あり。
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|speaker||Flore K. Kunst (Stockholm University / Freie Universitat Berlin)|
|title||Anatomy of Topological Flat and Surface States: Exact Solutions from Destructive Interference on Frustrated Lattices|
|abstract||The main feature of topological phases is the presence of robust boundary states, which appear for example in the form of chiral edge states in Chern insulators and open Fermi arcs on the surfaces of Weyl semimetals. Even though, non-interacting, topological systems can be straightforwardly described in fully periodic systems, the detail of the corresponding boundary states has mainly relied on numerical studies. In our work, we present a general method on how to find exact, analytical solutions for topological as well as trivial boundary states using a generic tight-binding model on a large class of geometrically frustrated lattices without the necessity of having to fine-tune hopping amplitudes. Our method is inspired by a similar approach that has been used in the past to construct, topologically-trivial, flat band models from local constraints on ‘line graphs’, in which case fine-tuning is required in the sense that hopping is strictly local. We expand on this work by considering a larger class of lattices, finding solutions for both topologically trivial and non-trivial bands, and going beyond the need for fine-tuning. In this sense, it is likely that our work will contribute to both the research fields of flat-band physics and that of topological matter, as well as advance the cross-fertilization between them. In my talk, I will present a number of examples to illustrate our discoveries, some of which are experimentally relevant such as the derivation of exact solutions for Fermi arcs in the recently synthesized slabs of pyrochlore iridates.|
|speaker||Dr. Emil J. Bergholtz (Stockholm University / Freie Universitat Berlin)|
|title||Fractional Chern insulators: From higher Chern number phases to non-Abelian twist defects|
|abstract||In this talk I will discuss fractional Chern insulators with emphasis on the analogy with more conventional continuum Landau level physics — and on aspects that are qualitatively new in the lattice setting such as Berry curvature fluctuations, competing instabilities and novel collective states of matter emerging in bands with higher Chern number. I will also explain how the lattice setting naturally allows for exotic extrinsic wormhole-like twist defects (aka “genons”) that effectively increase the genus of space.|