Schedule of winter semester (start from 14:45, Tuesday @ #933 (except when specified otherwise))
|speaker||Prof. Miguel A. Cazalilla (National Tsing Hua University, Taiwan)|
|title||Quantum Quenches in Fermion liquids: From the Generalized Gibbs Ensemble to Prethermalization|
One important paradigm of condensed matter physics is the notion of "adiabatic continuity" , namely,
the idea that the elementary excitations of a system can be put in one-to-one correspondence
with the excitations of a certain non-interacting model. The correspondence can be achieved by the
technical device of turning the (residual) interactions adiabatically. Good examples of the success of this
approach are Landau's theory of Fermi liquids and Haldane's theory of Tomonaga-Luttinger liquids.
In this seminar, I will consider the opposite limit, that is, a quantum quech in which the interactions are suddenly switched on. I will first discuss some (old) results  for a one dimensional Fermi gas (the so-called Luttinger model). Next, I will turn on to the two dimensional Fermi gas, for which I present recent results for a a long-range interaction quench in a spinless Fermi gas . For the latter, we have recently obtained the short to intermediate time dynamics using the method of bosonization of the Fermi surface. We have thus found that the asymptotic state predicted by bosonization is consistent with the prethermalized state that has been previously observed in numerical simulations of other fermion models. From the bosonized representation, we are able to explicitly construct the Generalized Gibbs Ensamble that describes the prethermalized state. Finally, I will also describe a protocol to perform an interaction quantum quench in a dipolar gas of Erbium atoms.
 P. W. Anderson, Basic Notions of Condensed Matter Physics, West View Press (1997).
 MAC, Phys. Rev. Lett. 97, 156403 (2006); A. Iucci and MAC, Phys. Rev. A 80, 063619 (2009).
 N. Nessi, A. Iucci, and MAC, in preparation.
|title||Magnetiztion process of a spin-1/2 distorted kagome magnet|
|abstract||Due to its strongly frustrated nature, the kagome antiferromanet (KAF) exhibits interesting features such as a localized magnon, magnetization plateaus, and a magnetization jump. Recently, the magnetization process of the spin-1/2 isotropic KAF has been determined accurately by a large-scale DMRG simulation, revealing new plateaus at some fractions of the saturated moment. However, extremely high magnetic fields are required to observe these new plateaus experimentally in candidate materials of KAFs. Recently, a new distorted kagome mineral has been synthesized, in which the interactions between neighboring spins are argued to be antiferromagnetic along one direction and ferromagnetic along the other two. We have investigated the magnetization process of such a distorted kagome magnet with mixed ferromagnetic and antiferromagnetic interactions through Lanczos diagonalization of finite-size clusters. We have identified robust plateaus and jumps in a reasonable range of magnetic fields that can be reached experimentally.|
|title||Adaptive Quantum State Estimation for Photons and its Efficiency|
|abstract||Adaptive quantum state estimation (AQSE) is a scheme of estimating the true value of the parameter that specifies the quantum state. In this scheme, the value is estimated repeatedly by POVMs, each of which is adapted to the state using past measurement outcomes. Recently, the experimental demonstration of AQSE for photons has been reported. In this experiment, the linear polarization of a single photon was used as a one-dimensional quantum statistical model, and its angle was estimated. In this talk, I will review this experiment and discuss its practical efficiency in comparison with some other schemes.|
|title||Tan relations and a possibility of their extension to p-wave systems|
Tan relations connect physical quantities and an asymptotic behavior of a correlation function, which hold universally for 2-component Fermi gases.
In such a system, the only relavant interaction is intercomponent s-wave scatterings, and a quantity called "contact" plays an essential role in the relation.
The situation significantly changes, however, when we consider a single component Fermi gas.
The Fermi statistics prohibits s-wave scatterings between particles and scatterings in a p-wave channel is a dominant interaction.
In this case, the centrifugal barrier requires a finite cutoff momentum scale, which makes the existence of universal relations like Tan's more nontrivial.
In this talk, I first review Tan relations of a 2-component Fermi gas. The origin of the asymptotic behavior of correlation functions is discussed. I then turn to the p-wave case, where I show seemingly non-universal nature of finite angular momentum systems and discuss a possibility of an extension of Tan relations to p-wave systems.
|title||Efficiency of the adaptive measurement|
|abstract||To estimate an unknown quantum state from measurement outputs, the quantum estimation theory has been developed. Cramer-Rao inequality, one of the central theorems in this theory, states that the accuracy of the estimation is limited by the Fisher information obtained by the measurement. However, since the POVM which maximizes the Fisher information depends on the unknown state itself, we cannot select the best POVM in advance. To resolve this difficulty, the adaptive measurement scheme has been proposed. In this scheme, we estimate the state from the already obtained data in each measurement step, and select the POVM which maximizes the Fisher information for the estimated state. Though the asymptotic properties of the adaptive measurement such as strong consistency and asymptotic efficiency have been investigated in detail, the accuracy of finite-time measurements has not been known. In this talk, I will discuss the efficiency of the adaptive measurement of a 1-qubit system. Numerical simulations suggest that the adaptive measurement is more efficient than the non-adaptive measurement for small number of measurements. However, the efficiency of adaptive measurement decreases for large number of measurements when the true state is pure.|
|title||Nonequilibrium Fluctuation Equalities Derived from Lebesgue's Decomposition|
|abstract||Nonequilibrium statistical mechanics have verified various types of fluctuation theorems in the last 20 years. However, integral fluctuation theorems so far cannot be applied to situations in which forward path-probability vanishes in a certain region or especially when error-free measurement is conducted under feedback control. We identify the mathematical origin of this problem by Lebesgue's decomposition and derive new nonequilibrium equalities applicable to the above two situations. Moreover, we find that the conventional fluctuation theorems are inapplicable under a new condition, where the new equalities remain still valid. Inequalities derived from the equalities impose a stronger restriction on the averaged entropy production than the conventional second law of thermodynamics in certain systems. The equalities and inequalities are confirmed by numerical simulations of overdamped Langevin systems.|
|title||Functional-renormalization study of universal three-body parameter|
Efimov state is a quite universal phenomena which appears in wide range of physical systems regardless of the energy scale of the system or the specific form of the inter-particle interaction. Actually it is known that the Efimov state appears in nuclear systems, atomic systems, magnons, excitons and so on.
One special character of the Efimov state is that it has the discrete scale invariance in the sense that the energy spectrum of the Efimov state forms a geometric series whose common ratio is an universal constant ~22.7. This nature actually ensures that the Efimov state is characterized only by two parameters: s-wave scattering length a which characterizes the strength of the inter-particle interaction, and the 3-body parameter a- (or κ*) which characterizes the ground-state energy of the Efimov state. Also, because of the discrete scale invariance, Efimov state is known to show the limit cycle behavior of the renormalization group.
Recently, experiments in the ultracold atoms showed that the value of the 3-body parameter a- becomes universal regardless of the atomic species, atomic internal states, and the applied magnetic field for Feshbach reonances. These results are quite surprising since the 3-body parameter was believed to be highly dependent on the details of the inter-particle interaction and therefore non-universal.
We have approached to this problem using a method called the functional-renormalization group and succeeded to clarify how this universal 3-body parameter can be understood in terms of the renormalization-group-limit-cycle behavior. We have also succeeded to reproduce the universal value of the 3-body parameter by evaluating the energy scale at which the limit-cycle behavior starts.
|title||Prethermalization in a coherently split one-dimensional Bose gas|
I will discuss the prethermalization dynamics of a coherently split one-dimensional Bose gas by using the Bethe ansatz method. Prethermalization is a relaxation process to a quasi-stationary state before reaching the true equilibrium state. The concept of prethermalization is important for understanding the fundamental aspects of quantum statistical mechanics such as “equilibration” and “relaxation” in isolated quantum many-body systems. Prethermalization and its connection to integrability in one-dimensional quantum systems have been intensively studied by both experiments and theories. For instance, M. Gring et al. recently observed the evolution of a rapidly and coherently split one-dimensional Bose gas for large numbers of particles and compare the evolution of the system to the prediction of the Tomonaga-Luttinger liquid (TLL) theory. Here we employ the Bethe ansatz method and precisely analyze the prethermalization process over a long-time scale beyond the TLL prediction.
References:  M. Gring et al. Science 337,1318 -1322(2012).
|title||Information flows and measurement bacactions in quantum continuous measurements|
From the beginning of the quantum theory,
the trade-off relations between measurement back-actions and acquired information
in the quantum measurement are discussed,
including uncertainty relations for uncommutative observable.
One way to quantify the back-actions of a quantum measurement is
to see the difference of the relative entropy of the given two quantum states
before and after the measurement.
The operational meaning of the relative entropy of two quantum states
is the distinguishability of the states---
two states are well-distinguished if their relative entropy is large.
By the measurement back-actions,
we obtain the information about the quantum state,
which can be used to distinguish the two states,
and the remaining quantum state is less distinguishable instead.
An exmaple is the projective measurement,
where the measurement output gives the information on an observable,
while the post-measurement state is the eigenstate of the observable
whick is the same irrespective of the pre-measurement state.|
In this talk, I will discuss the trade-off relation of the relative entropy of the measurement output and difference of the system diagonal relative entropy difference in quantum continous measurements. The conservation law of the relative entropy of the measurement output and the system relative entropy difference is shown in the photon-counting and quantum counting measurement. Finally I will present the general conditions for the quantum measuremen under which relative entropy conservation laws are valid for any pre-measurement states.
|speaker||Nguyen Thanh Phuc|
|title||Beliaev theory of spinor Bose-Einstein condensates and its applications|
The mean-field theoretical framework has succeeded in accounting for many phenomena observed in spinor Bose-Einstein condensates (BECs). However, there are special properties of spinor BECs that the mean-field theory fails to capture even qualitatively. This is because quantum fluctuations play an essential role in these features. |
In this talk, I will give a presentation of some of the special effects of quantum fluctuations on the phase structure and excitations of spinor BECs. They include fluctuation-induced metastability in first-order quantum phase transitions, emergent energy gap of quasi-Nambu-Goldstone modes, etc.
N. T. Phuc, Y. Kawaguchi and M. Ueda, Phys. Rev. A 88, 043629 (2013).
N. T. Phuc, Y. Kawaguchi and M. Ueda, Ann. Phys. 328, 158-219 (2013).
|title||Universal three-body parameter of the atomic Efiomv states|
The Efimov states are universal three-body bound states, which appear when the scattering length between the particles is resonantly large. The Efimov states feature their discrete scaling invariance, and they have recently been observed in ultracold atoms. In the Efimov physics, the so-called three-body parameter fixes the short-range phase and hence the scale of the energy spectrum. It has been long believed that the three-body parameter is sensitive to short-range details and therefore atomic species dependent. However, recent experiments have shown that the values of the three-body parameters stay fairly constant between different atomic species. I address this mystery of the universal three-body parameter, and reveal the physical origin of the universally for three identical bosons. Then, I discuss whether the same scenario may hold for other systems, such as 2 identical bosons/fermions + 1 distingushable atom. |
P. Naidon, S. Endo and M. Ueda, arXiv:1208.3912
|title||Integral quantum fluctuation theorems under measurement and feedback control|
After the mid 90's, fluctuation theorems and Jarzynski equalities have attracted considerable interest both experimentally and theoretically, since they give fundamental relations between physical quantities even in nonequilibrium processes. On the other hand, the effect of measurement and feedback control on thermodynamics has attracted many interests quite recently, due to the advancement of experimental techniques and the formulation of operational quantum measurement theory. It has been shown that the second law of thermodynamics should involve information contents characterizing the acquired knowledge of the system due to the measurement. It has also been shown that the information content gives the upperbound on the amount of work that can be extracted from the system beyond the conventional second law.|
In this talk, I will present the effect of a general quantum measurement and feedback control on thermodynamic systems and derive quantum fluctuation theorems for such a thermodynamic process. The information content that arises in the obtained equalities is given by the information gain, which takes into account the back action of the general measurement contrary to the classical case.
KF, Y. Watanabe, and M. Ueda, arXiv:1307.2362
|title||Magnetic structures of the spinor BEC induced by synthetic gauge fields|
I will discuss the possibility of controlling a magnetic structure of a spinor BEC by synthetic gauge fields confined in a tunable optical lattice. In this talk, I focus on a spin-1 BEC in an optical Kagome lattice. One can find interesting magnetic structures in the broken-axisymmetry phase. Such structures are not found in ferromagnetic and polar phases. The relative phase of condensate wavefunctions in the superfluid phase plays an important role in these magnetic structures.|
Recent experiments demonstrated the realization of the optical Kagome lattice , and of the synthetic gauge fields dependent on internal degrees of freedom . Our results will be also demonstrated with the use of existing experimental techniques. References:
 G.-B. Jo, J. Guzman, C.K. Thomas, P. Hosur, A. Vishwanath, and D.M. Stamper-Kurn, PRL 108, 045305 (2012).
 M. Aidelsburger, M. Atala, M. Lohse, J. T. Barreiro, B. Paredes and I. Bloch, arXiv:1308.0321 (2013).
|title||Spin-orbit-coupled atomic quantum gases|
In this talk, I will present our recent study on spin-orbit coupled atomic quantum gases: (1) we propose a general scheme for dynamically creating an atomic spin-orbit coupling, such as the Rashba or Dresselhaus types, for atoms with arbitrary hyperfine spin by using magnetic-field-gradient pulses; (2) we find that various types of vortices show up without rotation in the ground states of spin-orbit coupoled Bose-Einstein condensates as a direct consequence of spontaneous symmetry breaking into a combined gauge, spin, and space rotation symmetry. The combined symmetry also determines the vortex-core spin state.|
Zhi-Fang Xu, Li You, and Masahito Ueda, Phys. Rev. A 87, 063634 (2013).
Zhi-Fang Xu, Shingo Kobayashi, and Masahito Ueda, Phys. Rev. A 88, 013621 (2013).
|title||Entanglement spectra in topological phases and coupled Tomonaga-Luttinger liquids|
The entanglement spectrum (ES) has been found to provide useful probes of topological phases of matter and other exotic strongly correlated states.
For the system's ground state, the ES is defined as the full eigenvalue spectrum of the reduced density matrix
obtained by tracing out the degrees of freedom in part of the system.
A key result observed in various topological phases and other gapped systems has been the remarkable correspondence between the ES and the edge-state spectrum.
A similar correspondence has also been found in coupled one-dimensional systems such as spin ladders.
While this correspondence has been analytically proven for some topological phases,
it is interesting to ask what systems show this correspondence more generally and how the ES changes when the bulk energy gap closes. |
We here study the ES in two coupled Tomonaga-Luttinger liquids (TLLs) on parallel periodic chains. In addition to having direct applications to ladder systems, this problem is also closely related to the entanglement properties of two-dimensional topological phases. By expanding interchain interactions to quadratic order in bosonic fields, we are able to calculate the ES for both gapped and gapless phases using only methods for free theories. Based on the calculation for coupled chiral TLLs, we provide a simple proof for the correspondence between edge states and the ES in quantum Hall systems in consistency with previous numerical and analytical studies. In gapped phases of coupled non-chiral TLLs, we find that the ES consists of linearly dispersing modes. which resembles the spectrum of a single-chain TLL but is characterized by a modified TLL parameter. When the system becomes partially gapless (either in the symmetric or antisymmetric channel), we find an ES with an unusual dispersion relation proportional to the square root of the subsystem momentum.
Reference: R. Lundgren, Y. Fuji, SF, and M. Oshikawa, arXiv:1310.0829.
|speaker||Dr. Pietro Massignan (The Institute of Photonic Sciences, Barcelona)|
|title||Strongly-interacting quantum mixtures and quasi-2D Efimov physics|
In this talk I will first present the possible quasiparticles states of a few impurities in a Fermi gas, i.e., the polarons, molecules, and trimers, discussing their static and dynamic properties. I will then show how this information may be used to infer the phase diagram of strongly-interacting imbalanced Fermi mixtures, and to address the long-standing open issue of Itinerant Ferromagnetism.|
In the second part of the talk, I will present ongoing calculations on the fate of bosonic Efimov trimers in a quasi-2D geometry (i.e., in presence of a tight harmonic confinement in one direction).
|speaker||Prof. A. J. Leggett|
|title||Deceptively trivial looking problem concerning Bogoliubov quasiparticles|
|abstract||I ask the question what is the Berry's phase accumulated by a quasiparticle transported adiabatically around a simple vortex in a neutral s-wave Fermi superfluid.|
|speaker||Dr. Naoto Tsuji (Department of Physics, University of Tokyo)|
|title||Interaction quench dynamics in a fermionic superfluid|
Interaction quench, which can be realized in cold-atom experiments,
offers various fundamental
questions of interest, such as how the isolated quantum system
thermalize or not, and
whether any intermediate quasi-stationary state emerges after the
quench. For example,
in the fermionic Hubbard model without a long-range order, it is known
i.e., local observables quickly arrive at thermal values whereas the
Here we study the interaction quench for the fermionic Hubbard model with a long-range order . Especially we focus on the superfluid phase of the attractive Hubbard model. The time evolution is obtained by the nonequilibrium dynamical mean-field theory. We show that, contrary to the case in the normal phase, the order parameter does not prethermalize but stays to be a nonthermal finite value for a relatively long time even when the effective temperature exceeds the thermal critical temperature. It turns out that the transient dynamics (e.g., the Higgs mode) is governed by a "nonthermal critical point", which we discuss belongs to a universality class distinct from the conventional Ginzburg-Landau theory.
 N. Tsuji, M. Eckstein, and P. Werner, Phys. Rev. Lett. 110, 136404 (2013).
 P. Werner, N. Tsuji, and M. Eckstein, Phys. Rev. B 86, 205101 (2012).
 N. Tsuji and P. Werner, arXiv:1306.0307.
|speaker||Dr. Mile Gu (Centre for Quantum Technologies, Singapore)|
|title||Occam's Quantum Razor: How Quantum Mechanics can reduce the complexity of Classical Models|
Mathematical models are cornerstones of quantitative science. They take information available in the present, and use it to generate predictions about the future. They reflect our natural desire to rationalize the universe through cause and effect. Each model encapsulates our understanding of how future expectations depend on past observations. In the spirit of Occam's razor, simpler is better; should two models make identical predictions, the one that requires less input information is preferred.
Yet, for almost all stochastic processes, even the provably optimal classical models waste information. The amount of input information they demand exceeds the amount of predictive information they output. I this talk, I will show how to systematically construct quantum models that break this classical bound, and that the system of minimal entropy that simulates such processes must necessarily feature quantum dynamics. This indicates that many observed phenomena could be significantly simpler than classically possible should quantum effects be involved.
References: Nature Communications, 3, 762,
|speaker||Dr. Michael G. Endres (RIKEN)|
|title||Lattice study of unitary fermions in one spatial dimension|
|abstract||I will present results from a numerical study of unitary fermions confined to a harmonic trap and a finite box in one spatial dimension. The few- and many-body systems under consideration involve four-component fermions interacting via an attractive short-range four-body potential tuned to a resonance. These nonrelativistic systems are conformal and scale invariant up to a breaking due to the finite volume, and as such possess zero temperature properties which are qualitatively very similar to that of a unitary spin-1/2 Fermi gas in three spatial dimensions. I discuss the similarities of these two universal systems, and then present numerical results including evidence that a nonperturbative universal constant shared by both systems is in fact equal to within statistical uncertainties of 1%. I will finally discuss the potential implications of this finding.|
|speaker||Dr. Michael W. J. Bromley (University of Queensland)|
|title||One hundred years of stuff going around in circles|
The Sagnac effect was first demonstrated experimentally for light
one hundred years ago by French physicist Georges Sagnac and,
in recent years, atoms have begun to exhibit a rotation measurement
sensitivity able to go beyond that of light-based systems.
We have theoretically simulated an ultracold atom interferometer,
through two quantum-mechanical matter wavepackets,
e.g. Bose-Einstein condensates, that counter-propagate within
a rotating, circular ring-trap. We find that the accumulation
of the relative phase difference between wavepackets,
i.e. the Sagnac effect, is manifested as discrete phase jumps .
Textbook treatments would have you believe otherwise .
Our gedankenexperiments are straightforward to understand, and this
result has useful implications for the whole spectrum of matterwave
interferometry experimentalists and not just limited to atom optics.
 M. C. Kandes, R. Carretero-Gonzalez, M. W. J. Bromley, http://arxiv.org/abs/1306.1308
 `Atomic Physics', C.J. Foot (1st Edition, 2005)
|speaker||Prof. Miguiel A. Cazalilla (National Tsing Hua University, Taiwan)|
|title||Mixtures of Light Fermions and Heavy Bosons in Optical Lattices: Phase Equilibrium and Dissipative Quantum Phase Transitions|
Recent progress in producing mixtures of ultra-cold atoms has made it
possible to load species with very different atomic mass (like 6Li and
174Yb) in optical lattices (e.g. see Refs. [1,2]). In this talk, we
shall explore some of the possibilities offered by these new systems
for the quantum simulation of interesting many body phenomena [3,4].
In particular, we consider the situation in which the gases are loaded
in a sufficiently deep and anisotropic lattice which inhibits hoping
in two directions for the heavier bosons but not for the lighter
fermions. Under such conditions, the system should exhibit remarkable
stability against three-body losses for not too strong boson-fermion
We shall first report on a study of the miscibility of the fermions and bosons in such an optical lattice . The phase equilibria is studied within a mean-field approximation that accounts exactly for the boson-boson interactions, which, we argue, need to be treated non-perturbatively using the Bethe ansatz. In the second part of the talk , we shall discuss the nature Mott-Insulator transitions of the bosons in such a system , which are influenced by the presence of the fermions in two aspects: i) The fermions renormalize the lattice potential as well as the boson-fermion interactions ii) The fermions behave as a dissipative bath that can localize the bosons.
 H. Hara, Y. Takasu, Y. Yamaoka, J. Doyle, and Y. Takahashi, "Quantum degenerate mixtures of alkali and alkaline-earth-like atoms", Physical Review Letters 106, 205304 (2011).
 A. H. Hansen, A. Khramov, W. H. Dowd, Alan O. Jamison, Vladyslav V. Ivanov, and S. Gupta, arxiv: 1105.5751 (2011).
 E. Malatsetxebarria, F. M. Marchetti, and M. A. Cazalilla, "Phase Equilibrium of Binary Mixtures in Mixed Dimensions", arXiv:1304.6303 (2013).
 E. Malatsetxebarria, Zi Cai, U. Schollwoeck, and M. A. Cazalilla, "Dissipative Effects on the Superfluid to Insulator Transition in Mixed-dimensional Optical Lattices", arXiv:1305.1097 (2013).