|title||Observable-dependence of how random matrix theories can predict ETH corrections|
Understanding how isolated quantum systems thermalize has recently gathered renewed interest among theorists, thanks to the experimental realizations of such systems.
As a sufficient condition for the approach to thermal equilibrium, the eigenstate thermalization hypothesis (ETH) is particularly investigated, which focuses on matrix elements of observables in the thermodynamic limit.
Despite no proofs, many numerical studies suggest that the ETH holds true for few-body observables in nonintegrable systems.
Recent numerical studies also suggest that the ETH and its finite-size corrections coincide with the prediction of the random matrix theory (RMT).
This coincidence, which we call a quantum chaos conjecture (QCC), is expected to be the underlying mechanism for the ETH.
However, it is not completely understood to what extent the QCC is correct — especially how it depends on observables.
In this seminar, by investigating ETH corrections, we show that the QCC is true for a wide class of observables with various symmetries, including many-body operators. First, using the RMT, we predict a general form of ETH corrections, concerning the symmetries of systems and observables. We especially note that the ratios of variances between diagonal and off-diagonal matrix elements are universal and dependent only on the symmetries. Then, we numerically show that the ratios calculated in nonintegrable systems coincide with the prediction of the RMT, indicating the QCC, both for typical few-body and many-body observables. We finally remark, though, that a counterexample of the QCC exists even among simple observables, when it is related to the Hamiltonian.
|title||Coupling two quantum Hall states of bosons|
|abstract||We study the ground-state phase diagram of two-dimensional two-component (or pseudospin-1/2) Bose gases in synthetic magnetic fields in the space of the total filling factor and the ratio of the intercomponent coupling g↑↓ to the intracomponent one g>0. Using exact diagonalization, we show that when the intercomponent coupling is attractive (g↑↓<0), the product states of a pair of nearly independent quantum Hall states are remarkably robust and persist for |g↑↓| up to as large as g. This contrasts with the case of a repulsive intercomponent coupling, where a variety of spin-singlet quantum Hall states with high entanglement between the components emerge for g↑↓~=g. We interpret the marked dependence on the sign of g↑↓ in light of Haldane's pseudopotentials on Haldane's sphere, and also argue that a qualitatively reversed dependence on the sign of g↑↓ occurs in two-component Bose gases in mutually antiparallel magnetic fields possessing time-reversal symmetry.|
|title||Time Complexity of Hamiltonian Estimation Problem|
Estimation of unknown parameter in a dynamical systems is called quantum metrology and has been well studied for one-parameter dynamics.
This includes the separation between standard limit and Heisenberg limit, where entanglement squares the efficiency of estimation.
However, not so much is known about multi-parameter case, especially when there are large number of parameters. |
We set a up a generalized class of quantum metrology problem, in which one aims to estimate m-parameter Hamiltonian on N-dimensional Hilbert space. As a result, we found that quantum-mechanical cost contributes to the precision square as much as classical cost, in analogy with standard and Heisenberg limits. Moreover, we have established a rigorous lower bound of estimation time in order to achieve certain estimation error. In case the parameter meets a condition we call isotropy, the lower bound can be given in terms of Hilbert space dimension N, rather than degree of freedom m. As a consequence it is implied that difficulty of estimation is asymptotically the same, whether or not we know eigenstates in prior.
|speaker||Nguyen Thanh Phuc|
|title||Geometrically Frustrated Coarsening Dynamics in Spinor Bose-Fermi Mixtures|
Coarsening dynamics theory has successfully described the equilibration of a broad class of systems. By studying the relaxation of a periodic array of microcondensates immersed in a Fermi gas which can mediate long-range spin interactions to simulate frustrated classical magnets, we show that coarsening dynamics can be suppressed by geometrical frustration . The system is found to eventually approach a metastable state which is robust against random field noise and characterized by finite correlation lengths with the emergence of topologically stable Z2 vortices. We find universal scaling laws with no thermal-equilibrium analog that relate the correlation lengths and the number of vortices to the degree of frustration in the system.|
 arxiv: 1609.03015
|speaker||Kohaku So (Hongbo Zeng)|
|title||Phases and phase transitions of ferromagnetic spin-1 bosons in optical lattices|
Cold atoms in optical lattices are considered to be ideal arenas for exploring quantum many-body physics. In particular, cold atoms with spin degrees of freedom have gained considerable interests because of the possibility they offer of modelling quantum magnetism and exploring the interplay between spatial and spin degrees of freedom.|
Although antiferromagnetic spinor bosons have been widely studied in this context, primarily because of their peculiar properties, the ferromagnetic counterpart is not extensively studied. However, previous mean-field analyses of the systems without lattices suggest that the competition between the ferromagnetic interaction and the quadratic Zeeman effect can give rise to interesting phenomena.
We have studied phase diagrams of spin-1 ferromagnetic Bose-Hubbard model under the quadratic Zeeman effect. Using the decoupling approximation, we obtained ground state and non-zero temperature phase diagrams, where, in addition to the Mott-insulator, two distinct superfluid phase appear. Moreover, we found that for sufficiently low temperatures, phase transitions between these two superfluid phases are discontinuous at some part of the phase boundary, which is in clear contrast to the system without lattice, where the corresponding phase transition is known to be continuous.
|title||Anomalous conductance in a cold Fermi gas|
|abstract||Two-terminal setup realized in cold atoms offers a unique platform to investigate transport properties in an attractively-interacting Fermi gas. Here, we analyze the conduction properties through a quantum point contact in a superfluid fluctuation regime. We show that in a ballistic regime where the quantization of the conductance emerges in the absence of an interaction, the contact resistance is strongly renormalized by superfluid fluctuations in reservoirs. Our results are consistent with recent experimental results at ETH.|
|title||Topology of a limit cycle and universal four-body bound states in Efimov physics|
Resonantly interacting three-particles universally forms an infinite series of self-similar Efimov trimers. While the universality in critical phenomena is elucidated by a renormalization-group fixed point, the universality of the Efimov effect is, due to its self-similarity, characterized by a renormalization-group limit cycle which refers to a periodic renormalization-group flow.|
Recently, four-body extension of the Efimov effect was extensively investigated to reveal universal two tetramers accompanying an Efimov trimer at the unitarity limit. In this presentation, a close connection between a topological property of the limit cycle and a universal feature of four-body Efimov physics is presented. A signature of a topological phase transition for mass-imbalanced four-body systems is also suggested.
|title||Exact out-of-time-ordered correlation functions for an interacting lattice fermion model|
Out-of-time-ordered correlation (OTOC) function is a useful measure that diagnoses chaos (scrambling) and information spreading in quantum
many-body systems. The interest in OTOCs is recently generated by the studies of the Sachdev-Ye-Kitaev model and AdS/CFT correspondence. Inthis talk, I will present our recent results on exact solutions for OTOCs in an interacting lattice fermion model, namely the Falicov-Kimball model . Our approach is based on the generalization of the nonequilibrium dynamical mean-field theory to an extended Kadanoff-Baym contour. I also discuss an experimental protocol to measure OTOCs in ultracold atomic systems.|
 N. Tsuji, P. Werner, and M. Ueda, arXiv:1610.01251 .
|title||Zeno Hall effect|
|abstract||The quantum Zeno effect and the Hall effect are two fundamental physical phenomena. In this seminar, we show that the former can give rise to the latter. For a two-dimensional lattice, we use the continuous quantum Zeno effect to tailor the Hilbert space into a single Bloch band with nonzero Berry curvature. Even in the absence of kinetic degree of freedom for the bare lattice system, anomalous wave-packet dynamics emerges in response to a potential gradient. We call such a phenomenon the Zeno Hall effect, which can be regarded as a Hall effect "without Hamiltonian". Our scheme provides a general protocol to engineer flat bands. A possible experimental implementation with cold atoms in an optical lattice is discussed.|
|title||Einstein de-Haas effect in a dipolar Fermi gas|
|abstract||Recent experimental progress in trapping rare-earth atoms such as Dysprosium make it possible to study quantum gases with strong magnetic dipole-dipole interactions. Dipole-dipole interactions are long-ranged, anisotropic and do not conserve total spin, meaning that magnetization of a dipolar quantum gas is not conserved. However, since total angular momentum, the sum of spin and orbital angular momentum must still be conserved, dipole-dipole interactions can convert magnetization into center of mass orbital angular momentum. This manifests in the simultaneous de-magnetization and rotation of the gas. This transfer is analogous to the Einstein-de Haas effect, where a suspended ferromagnetic metal cylinder rotates when its magnetization is reversed, and has been predicted to occur in a dipolar BEC. I will show that the same effect occurs also in a dipolar Fermi gas and dicuss its robustness regarding external magnetic fields and short-range interactions.|
|speaker||Shuhei M. Yoshida|
|title||Universal properties of p-wave Fermi gases in three and two dimensions|
|abstract||In gaseous systems with resonant interactions, there are universal relations which bridge short-range few-body correlations to the bulk properties. A quantity called the contact plays a key role, characterizing both the short- and long-range properties. They are known to hold quite generally, whether the system is in the ground state or at a finite temperature, in the superfluid or normal fluid phase, and so on. We study such universal relations in p-wave Fermi gases in three dimensions and demonstrate that a nematic component of the contact emerges in the absence of the axisymmetry even in the normal fluid phase. We also discuss the prospect for universal relations in p-wave Fermi gases in two dimensions, where the three-body correlations are expected to be significant.|
|title||Fluctuation Theorems in Feedback-controlled Open Quantum Systems: Interplay between Coherence and Absolute Irreversibility|
|abstract||We consider stochastic thermodynamics in open quantum systems under feedback control and drive fluctuation theorems in the presence of absolute irreversibility, which is singularly strong irreversibility with divergent entropy production. In the conventional approaches to derive the fluctuation theorems under feedback control, no measurement is needed in the backward process. In contrast, in our system, we show that the backward process involves a measurement and post-selection to take into account the back-action of the forward measurement. As a consequence, we derive fluctuation theorems in the presence of absolute irreversibility. While in the conventional approaches a projective (error-free) measurement ordinarily leads to emergence of absolute irreversibility, in our formulation absolute irreversibility is suppressed due to quantum coherence. This is because quantum coherence allows the state of the system to instantaneously expand to entire Hilbert space. Moreover, we find that absolute irreversibility emerges even in our system when we turn off coherent feedback after the measurement. Thus, the notion of absolute irreversibility is closely linked to the existence of quantum coherence.|
|speaker||Prof. Shina Tan (Georgia Tech)|
|title||Short-range correlations of one-dimensional quantum gases|
|abstract||The correlation function is an important quantity in the physics of ultracold quantum gases because it provides more information about the quantum many-body states than the simple density profiles. The contact of the three-dimensional quantum gases with large scattering length, for instance, is directly related to the short-range density-density correlation. We studied the M-body local correlator, g_M, of the one-dimensional strongly repulsive Bose gas within the Lieb-Liniger model. g_M is the probability of finding M bosons at the same point. We calculated g_M to the sub-leading order in powers of 1/gamma, where gamma is Lieb-Liniger's dimensionless coupling parameter. For M=2 and 3 our results reproduce the known expressions for g_2 and g_3 in the literature. We also found the leading order expression of the short-distance NON-LOCAL M-body correlation functions in terms of the wave function of M bosons at zero collision energy.|
|title||Non-conservation of topological charges in multiple topological excitations|
Conservation of topological charges is of fundamental importance for topological excitations. One unique feature of coexistences of topological excitations is that topological charges of individual topological excitations are no longer conserved [1,2]. For example, in coexistence of a half-vortex and a monopole in a uniaxial nematic liquid crystal, the topological charge of the latter changes its sign when it makes a complete circuit of the former. However, the topological charge is conserved as an entire system in all examples known so far such as coexistences of vortices and those of vortices and monopoles [3,4]. |
Here, we find that in coexistences of vortices and skyrmions, the total charge of skyrmions can change. One unique feature of the topology of this coexistence is no longer characterized by conventional homotopy groups, but by the topological invariant of textures on a two-dimensional torus. We develop a general method to calculate this and show that the parity of the total charge of skyrmions is a new invariant. Through classifying all types of coexistence of topological excitations up to three dimensions, we find that the topological charge for three-dimensional skyrmions is not conserved in the presence of two skyrmion lines in three dimensions, from which we predict that a knot with linking number two can be created continuously upon crossing two skyrmion lines with unit charge in a chiral magnet.
 M. Kleman, L. Michel, and G. Toulouse, J. Phys. Lett. (Paris) 38 L-195(1977).
 G. E. Volovik and V. P. Mineev, Zh. Eksp. Teor. Fiz 72, 2256 (1977).
 M. Bucher, H. Lo, and J. Preskill, Nucl. Phys. B 386, 3 (1982).
 S. Kobayashi, N. Tarantino, and M. Ueda, Phys. Rev. A 89, 033603 (2014).
|title||Measuring quantum Fisher information and skew information through linear response functions|
|abstract||The quantum Fisher information is characterized as metrics on the space of states that decrease under information processing monotonically, and is applied to various fields such as quantum information theory and condensed matter physics. However, the operational meaning of many types of the quantum Fisher information beyond the monotonicity has not been investigated in a unified manner, and even how they are related to measurable quantities has not been clarified yet. We discuss the relation between the general quantum Fisher information and measurable quantities in linear response theory. For that purpose, we generalize the fluctuation-dissipation theorem that shows the relation between the linear response functions and generalized covariances. Based on this theorem, we show that the quantum Fisher information can be determined by measuring the admittance or the dynamical susceptibility. As an application, we propose a method to verify skew information-based uncertainty relations experimentally.|
|speaker||Soonwon Choi (Havard)|
|title||Non-equilibrium many-body spin dynamics in diamond|
In this talk, we will discuss two recent developments in non-equilibrium quantum dynamics of strongly interacting many-body systems: I. critically slow thermalization in a disordered dipolar spin ensemble  and II. the observation of discrete time crystalline order . Both of these experiments were enabled by a high density ensemble of nitrogen-vacancy (NV) color centers in diamond . As a mixture of theory and experiments, the talk will be self-contained and pedagogical, reviewing some of basic concepts in many-body localization, Floquet time-crystal, spin properties of NV centers and experimental techniques to manipulate and engineer the dynamics.|
Part I: Statistical mechanics underlies our understanding of macroscopic quantum systems. It is based on the assumption that out-of-equilibrium systems rapidly approach their equilibrium states, forgetting any information about their microscopic initial conditions. This fundamental paradigm is challenged by disordered systems, in which a slowdown or even absence of thermalization is expected. By controlling the spin states of the ~10^6 NV centers, we observe slow, sub-exponential thermalization consistent with power laws that exhibit disorder-dependent exponents; this behavior is modified at late times owing to many-body interactions. These observations are quantitatively explained by a resonance counting theory that incorporates the effects of both disorder and interactions.
Part II: The interplay of periodic driving, disorder, and strong interactions has recently been predicted to result in exotic ``time-crystalline'' phases, which spontaneously break the discrete time-translation symmetry of the underlying drive. We report the experimental observation of such discrete time-crystalline order and the observation of long-lived temporal correlations at integer multiples of the fundamental driving period. We experimentally identify the phase boundary and find that the temporal order is protected by strong interactions; this order is remarkably stable against perturbations, even in the presence of slow thermalization. We provide a theoretical description of approximate Floquet eigenstates of the system based on product state ansatz and predict the phase boundary, which is in qualitative agreement with our observations.
 G. Kucsko et al, arXiv:1609.08216 .
 S. Choi et al, arXiv:1610.08057 .
 J. Choi et al, arXiv:1608.05471 .
|title||Fast generation of macroscopic superposition states by coherent driving|
|abstract||Macroscopic superposition states (MSSs) of spins or qubits are potentially applicable to precision measurements, quantum computation and communication technologies, besides being of fundamental interests. Such macroscopic and highly entangled states can be generated via quadratic interactions such as one-axis twisting (OAT); however, the time needed to create an MSS is often nonnegligible compared with coherence time of a spin ensemble. In this presentation, we propose a new method to create high-fidelity MSSs consisting of the OAT and a coherent driving field. In our method, the preparation time of an MSS for N>=350 spins is estimated to be shorter than the coherence times of an 87Rb Bose-Einstein condensate and an ion-trapped 9Be+ observed in recent experiments. The speedup on the MSS generation time can be a significant advantage of this scheme to experimentally generate and test these states. These numbers are promising for relatively large spin ensembles to form a MSS with the current technology.|
|title||Coarsening dynamics of the spin-1 spinor Bose-Hubbard model in the one-dimensional system|
In the context of thermalization, non-equilibrium phenomena such as pre-thermalization, non-thermal fixed point, turbulence, etc., are actively studied in atomic gases. Recently, as the thermalization dominated by topological defects, the coarsening dynamics in the atomic gas is investigated by numerical calculations of the Gross-Pitaevskii (GP) model, where the dynamical scaling and the characteristic domain growth laws are found [1, 2]. These studies neglect the quantum fluctuation and most of results are similar to the classical systems.
We expect that the atomic gas gives us the stage to study the coarsening dynamics with the quantum fluctuation, which has possibility to exhibit phenomena not found in the classical systems. To investigate the coarsening dynamics affected by the quantum fluctuation, we study the dynamics of the spin-1 spinor Bose-Hubbard model in the one-dimensional system. In the seminar, we show analytical and numerical results obtained recently.|
 J. Hofmann, S. S. Natu, and S. D. Sarma, Phys. Rev. Lett. 113, 095702 (2014).
 L. A. Williamson and P. B. Blakie, Phys. Rev. Lett. 116, 025301 (2016).
|title||Introduction to vortex lattices|
|abstract||By rotating Bose einstein condensate in a magnetic trap, vortex lattices are obsereved experimantally both in 1 component and 2 components. Statistical feature of vortex lattices is explained by Gross-Pitaevskii (GP) equation and Bogoliubov-de Gennes (BdG) equation. On the other hand, dynamical feature of vortex lattices are obtained from hydrodynamic approach and BdG eq, and one of the example of dynamical feature of vortex lattices is Tkachenko mode. By using GP eq, we find a vortex lattice structure changes for 2 components condensate. Also Tkachenko mode is studied by using hydrodynamic approach and this explains the experiment very well.|
|title||Many-body interferometry of magnetic polaron dynamics|
Recent experimental realizations of imbalanced mixtures of ultracold atoms have opened up new possibilities for studying impurity physics in a highly controlled manner. An impurity is coupled to collective excitations of surrounding atoms and forms a quasiparticle excitation known as a polaron. Owing to an ability to tune the impurity-bath interaction, cold atomic systems can potentially give new insights in polaron physics beyond the conventional Frolich paradigm. |
In this talk, we demonstrate that an impurity immersed in a two-component Bose-Einstein condensate provides a unique platform to study the real-time dynamics of magnetic polaron. We show that the formation of magnetic polaron triggers characteristic many-body spin dynamics in the bosonic bath, which can directly be measured by Ramsey interference. We employ time-dependent variational approach to investigate the spin dynamics, and find that the underlying few-body bound states induce the oscillatory behavior in the Ramsey signals. Our results suggest a novel approach to studying few-body physics through the many-body dynamics, making a sharp contrast to the conventional spectroscopic measurements. A possible experimental implementation of our system is also discussed.
 Y. Ashida, R. Schmidt, L. Tarruell, and E. Demler, arXiv:1701.01454 .
|title||Superfluidity of monolayers of 4He|
|abstract||The experiment which we are going to do from this spring is to find superfluidity of monolayers of 4He. Supersolid, the state which has both superfluidity and crystalline order, has been theoretically predicted since 1960s. In 2004, torsional oscillator (TO) measurement by Kim and Chan detected the drop of resonancet periods, which seemed to imply the nonclassical rotational inertia and hence superfluidity. However, it was later proved that the observed drop of the resonance periods was due to the low-temperature shear modulus, not supersolidity. So far, supersolid has not been found yet. TO experiments on the monolayers of Helium were also done by some groups. From some thermodynamic experiments and 2D theory, a phase called C2 phase (ρ~20 nm-3) is expected to be a crystalline phase with superfluidity. In this seminar, I will present an overview of the theories and experiments of supersolid, and explain a plan of our experiment.|
|speaker||Niklas Mann (Univ. Hamburg)|
|title||Nonequilibrium Quantum Dynamics of Hybrid Systems|
The combination of different physical systems is motivated by the idea to exploit advantages and a higher controllability. Here, atomic/molecular and optical (AMO) physical systems are combined to form a hybrid quantum system. More precisely, an atomic condensate is coupled to a mechanical membrane whose mass exceeds the mass of the atomic condensate by ten orders of magnitude.The coupling between both parts is mediated by a coherent laser light field that allows a long distance coupling. Different schemes allow to couple the vibrational mode of the macroscopic solid-state system either to the motional degree of freedom [1,2] or to the internal states  of the atoms in the condensate. As an application sympathetic cooling with ultracold atoms enables ground-state cooling of the membrane in a regime where this by purely optomechanical techniques cannot be reached .|
In this talk, I will outline the theoretical background, recent experimental progress and further possible applications.
 B. Vogell, K. Stannigel, P. Zoller, K. Hammerer, M.T. Rakher, M. Korppi, A. Jöckel, P. Treutlein, Phys. Rev. A 87, (2013) 023816.
 A. Jöckel, A. Faber, T. Kampschulte, M. Korppi, M.T. Rakher, P. Treutlein, Nature Nanotech. 10 (2014) 55.
 B. Vogell, T. Kampschulte, M.T. Rakher, A. Faber, P. Treutlein, K. Hammerer, P. Zoller, New J. Phys. 17, (2015) 043044.
|speaker||Prof. Jörg Schmiedmayer (TU-Wien)|
|title||Does an isolated quantum system relax?|
The evolution of an isolated quantum system is unitary. This is simple to probe for small systems consisting of few non-interacting particles. But what happens if the system becomes large and its constituents interact? In general, one will not be able to follow the evolution of the complex many body eigenstates. |
Ultra cold quantum gases are an ideal system to probe these aspects of many body quantum physics and the related quantum fields. Our pet systems are one-dimensional Bose-gases. Interfering two systems allows studying coherence between the two quantum fields and the full distribution functions and correlation functions give detailed insight into the many body states and their non-equilibrium evolution.
In our experiments we study how the coherence created between the two isolated one-dimensional quantum gases by coherent splitting slowly degrades by coupling to the many internal degrees of freedom available . We find that a one-dimensional quantum system relaxes to a pre-thermalisatized quasi steady state  which emerges through a light cone like spreading of `de-coherence' . The pre-thermalized state is described by a generalized Gibbs ensemble . Finally, we investigate the further evolution away from the pre-thermalized state. On one hand we show that by engineering the Quasiparticles we can create many body quantum revivals. On the other hand, we point to two distinct ways for further relaxation towards a final state that appears indistinguishable from a thermally relaxed state. The system looks like two classically separated objects. This illustrates how classical physics can emerge from unitary evolution of a complex enough quantum system. We conjecture that our experiments points to a universal way through which relaxation in isolated many body quantum systems proceeds if the low energy dynamics is dominated by scrambling of the eigenmodes of long lived excitations (quasi particles) .
Supported by the Wittgenstein Prize, the Austrian Science Foundation (FWF) SFB FoQuS: F40-P10 and the EU through the ERC-AdG QuantumRelax.
 S. Hofferberth et al. Nature, 449, 324 (2007).
 M. Gring et al., Science, 337, 1318 (2012); D. Adu Smith et al. NJP, 15, 075011 (2013).
 T. Langen et al., Nature Physics, 9, 640–643 (2013).
 T. Langen et al., Science 348 207-211 (2015).
 T. Langen, T. Gasenzer, J. Schmiedmayer, J. Stat. Mech. 064009 (2016).
|speaker||Dr. Shimpei Endo (School of Physics & Astronomy, Monash University)|
|title||From few-body to many-body physics in a strongly interacting quantum gas|
|abstract||In recent cold atom studies, there has been growing interest in universal few-body and many-body physics in the strongly interacting regime. In this seminar, I show how quantum few-body studies can help understanding quantum many-body problems. I first talk about the virial expansion in the unitary Fermi and Bose gas. I show that the 3rd and 4th virial expansion coefficients can be accurately calculated by solving 3-body and 4-body problems, reproducing the 3rd and 4th virial coefficients reported in the unitary Fermi gas experiments. Secondly, I show how few-body knowledge can help predicting novel quantum phase. I show that three-component Fermi gas composed of universal trimers can appear in a two-component mass-imbalanced Fermi gas. We find that the trimer-trimer interaction is repulsive, suggesting that this trimer phase can be stably realized in cold atom mixtures, such as 53Cr-6Li.|