Pushing the Boundaries with Cold Atoms

21 Jan 2013, 08:00 → 15 Feb 2013, 18:00 Europe/Stockholm

132:028 (Nordita)

Chris Pethick (NBI and Nordita), Emil Lundh (University of Umeå), Jani-Petri Martikainen (Nordita), Jonas Larson (Stockholm University), Päivi Törmä (Aalto University)

Venue

Nordita, Stockholm, Sweden

Scope

The ability to cool, coherently manipulate and measure atomic gases make these systems good candidates for studies of quantum many-body phenomena. Current experimental techniques allow for a plethora of different setups to be studied. This four week program focuses on new directions within the field of ultracold atomic gases. Interesting topics include for example,

Synthetic gauge fields for cold atoms. Multi-component atoms, spinor condensates. Non-equilibrium physics, quantum thermalization... Exotic states in optical lattices: topological matter, p-band physics… Disorder. Light-matter interface, cold atoms in optical cavities. Dipolar and Rydberg gases. Bistability, non-linearity, optomechanics with cold gases.

We aim to bring together theoreticians as well as experimentalists. Many of the above mentioned topics have seen experimental breakthroughs during the last few years, and we hope to hear more about this during the program. At the same time, since our field evolves rapidly, the list above is most likely to change somewhat before the program starts.

As a monthly program, the idea is that participants get the opportunity to get to know each other and establish contacts. Thereby, we strongly encourage the participants to stay at least for one week.

[Timetable - available from start of the program]

Format

There will be only few talks per day with plenty of time for discussions in a relaxed atmosphere.

Invited Participants (preliminary)

Alexander Altland (University of Köln, Germany) Immanuel Bloch (University of München, Germany) Georg Bruun (Aarhus University, Denmark) Jean Dalibard (CNRS, France) Jens Eisert (Freie Universität Berlin, Germany) Andre Eckardt (Max Planck Institute Dresden, Germany) Tilman Esslinger (ETH, Zürich, Switzerland) Axel Griesmaier (University of Stuttgardt, Germany) Rudolf Grimm (University of Innsbruck, Austria) Andreas Hemmerich (University of Hamburg, Germany) Randy Hulet (Rice University, USA) Massimo Inguscio (University of Florence, Italy) Adrian Kantian (University of Geneva, Switzerland) Karyn Le Hur (Ecole Polytechnique Palaiseau, France) Maciej Lewenstein (ICFO, Barcelona, Spain) Vincent Liu (University of Pittsburgh, USA) Boris Malomed (Tel Aviv University, Israel) Cristiane Morais Smith (Utrecht University, Netherlands) Silke Ospelkaus (Leibniz Universität Hannover, Germany) Stephanie Reimann (University of Lund, Sweden) Luis Santos (Leibniz Universität Hannover, Germany) Achim Schwenk (Technical University of Darmstadt, Germany) Klaus Sengstock (University of Hamburg, Germany) Ian Spielman (NIST, Washington, USA) Henk Stoof (Universiteit of Utrecht, Netherlands) Masahito Ueda (University of Tokyo, Japan) Peter Zoller (University of Innsbruck, Austria) Wilhelm Zwerger (Technische Universität München, Germany)

Application

The registration is not yet open, but eventually if you want to apply for participation in the program, please fill in the application form. You will be informed by the organizers shortly after the application deadline whether your application has been approved. Due to space restrictions, the total number of participants is strictly limited.

Application deadline: 8 November 2012

A minimum stay of one working week is required and we encourage participants to stay for a period of at least two weeks.

There is no registration fee.

Travel Reimbursement

PhD students and young Postdoc fellows are eligible for travel grants to participate in the program. If you are interested in such a grant, please mark the corresponding field in the application form, briefly summarize your interest in the program in the comments field, and indicate an estimation of your expected travel expenses. Since only a limited number of grants is available, decision concerning the grants will be made on a case-by-case basis and you will be notified shortly after the application deadline.

Accommodation

Nordita provides a limited number of rooms in the Stockholm apartment hotel BizApartments free of charge for program participants.

Sponsored by:

Monday, 21 January

10:30 → 10:50 Welcome!

pictures Pushing_the_Boundaries_with_Cold_Atoms_(21_January_2013_-_15_February_2013).png

10:50 → 11:35 Parity violating superfluidity in ultra-cold fermions under the influence of artificial non-Abelian gauge fields

I discuss the creation of parity violating Fermi superfluids in the presence of non-Abelian gauge fields involving spin-orbit coupling and crossed Zeeman fields. I focus on spin-orbit coupling with equal Rashba and Dresselhaus (ERD) strengths which has been realized experimentally in ultra- cold atoms, but also discuss the case of arbitrary mixing of Rashba and Dresselhaus (RD) and of Rashba-only (RO) spin- orbit coupling. To illustrate the emergence of parity violation in the superfluid, I analyse first the excitation spectrum in the normal state and show that the generalized helicity bands do not have inversion symmetry in momentum space when crossed Zeeman fields are present. This is also reflected in the superfluid phase, where the order parameter tensor in the generalized helicity basis violates parity. However, the pairing fields in singlet and triplet channels of the generalized helicity basis are still parity even and odd, respectively. Parity violation is further reflected on ground state properties such as the spin-resolved momentum distribution, and in excitation properties such as the spin-dependent spectral function and density of states.

Speaker: Prof. Carlos Sa de Melo (Georgia Tech (USA))

13:45 → 14:00 History of NORDITA

Speaker: Prof. Chris Pethick (NORDITA)

Slides NorditaHistory2013.pdf

14:00 → 14:45 Correlated Topological Phases: From Condensed-Matter Systems to Artificial Gauge Fields

During the last decade, experiments have established the existence of unconventional states of matter in a variety of low-dimensional quantum systems. This includes equilibrium states characterized by topological properties as well as stationary states in and out of equilibrium situations. In this Talk, we focus on topological phases of matter, their experimental signatures, and possible ways of utilizing them as platforms for topologically protected quantum computation. With the important progress on the quantum control of light-matter interaction, one can now also engineer very tunable artificial complex quantum networks. We pedagogically introduce novel topological phases in correlated materials and artificial quantum networks, such as in cavity/circuit QED systems and cold atoms.

Speaker: Prof. Karyn Le Hur (Ecole Polytechnique Palaiseau)

15:00 → 16:00 Coffee break

Daily coffee break

Tuesday, 22 January

10:00 → 10:45 Long-lived repulsive and attractive polarons in a strongly interacting Fermi gas.

I discuss the existence of long-lived repulsive as well as attractive polarons in a strongly interacting Fermi gas. The energy, lifetime, and quasiparticle residue of the polarons are calculated, and I show show how they accurately describe experimental data. Finally, I discuss possible consequences of these results regarding observing itinerant ferromagnetism in atomic gases

Speaker: Prof. Georg Bruun (Aarhus University)

Slides BruunTalk.pdf

14:00 → 14:30 Finite temperature phase diagram of a spin-polarized Fermi gas in a dimensional crossover

We investigate exotic paired states of spin-polarized Fermi gases in the dimensional crossover between onedimensional and three-dimensional optical lattices. We compute the finite temperature phase diagram of the system along the dimensional crossover using real-space dynamical mean-field theory in combination with the continuous-time auxiliary field quantum Monte Carlo method. We find that the Fulde-Ferrell–Larkin-Ovchinnikov (FFLO) state can be realized for a broad range of parameters throughout the dimensional crossover. We study the melting of the FFLO state which involves the formation of a shell structure with a strong dependence on the dimensionality. Moreover, we investigate how the spectral function of the system is modified by the non-trivial spatial structure of the FFLO state.

Speaker: Mr Miikka Heikkinen (Aalto University)

14:30 → 15:00 Stability of spin liquid phases of alkaline earth atoms at Finite temperature

We describe the spin dynamics of the antiferromagnetic Mott insulator ground state of high spin fermions on a 2- dimensional hexagonal lattice. It was pointed out that such multicomponent systems in 1 and 2 dimensions can realize states without breaking the spin rotation symmetry when the number of components is large enough [1-3]. The low energy fluctuations on top of these so called spin liquid states are described by various gauge theories whose character depend on the symmetries of the mean-field solution [4]. Therefore high spin, ultracold, fermionic alkaline earth metal atoms loaded into optical lattices can serve as simulators of quantum gauge theories. Since in experiments with ultracold atoms it is a hard task to go to sufficiently low temperatures it becomes important to study the effects of finite temperature. We carry out the stability analysis of the mean-field solution and calculate the free energy at Finite temperature to determine the phase diagram relevant for experiments. References: 1. J. B. Marston, and I. Affleck, Phys. Rev. B 39, 11538, (1989). 2. M. Hermele, V. Gurarie, and A. M. Rey, Phys. Rev. Lett. 103, 135301 (2009). 3. G. Szirmai, E. Szirmai, A. Zamora, and M. Lewenstein, Phys. Rev. A 84, 011611 (2011). 4. X.-G. Wen, Quantum Field Theory of Many-Body Systems (Oxford University Press, 2004).

Speaker: Dr Gergely Szirmai (Wigner Research Centre of the Hungarian Academy of Sciences, Budapest)

Slides nordita_szirmai.pdf

15:00 → 16:00 Coffee break

Daily coffee break

Wednesday, 23 January

10:00 → 10:45 Ultracold atoms and neutron-rich matter in nuclei and in astrophysics

Speaker: Prof. Achim Scwenk (TU Darmstadt/EMMI)

Slides NORDITA_2013.pdf

14:00 → 14:45 Mobile impurities within a many-body quantum system

Advances in cold gases physics are enabling experiments involving the direct manipulation and observation of single- or few-atom mobile impurities [1,2] within a many-body quantum system, a topic of longstanding interest for condensed matter theory, where it is related to studies of e.g. conductivity and the X-ray edge problem. In light of these developments we study the dynamics of single mobile impurities in 1D quantum liquids, using analytical and DMRG techniques. We address the question of whether the recently proposed subdiffusive regime of impurity motion [3] constitutes a novel universality class of single particle excitations, one that is very different from that of the standard Tomonaga- Luttinger liquid excitations. We study the conditions for observing this regime and its' crossover to the ballistic regime. We furthermore examine the possibilities to observe the intermediate diffusive motion of impurities in these systems as well as various types of polaronic dynamics [2,4]. [1] J. Catani, G. Lamporesi, D. Naik et. al., Phys. Rev. A 85, 023623 (2012) [2[ T. Fukuhara, A. Kantian, M. Endres et. al., arXiv:1209.6468 [3] M. B. Zvonarev, V. V. Cheianov, T. Giamarchi, PRL 99, 240404 (2007); PRL 103, 110401 (2009) [4] F. Massel, A. Kantian, A. J. Daley et. al., arXiv:1210.4270

Speaker: Dr Adrian Kantian (University of Geneva)

15:00 → 15:30 Coffee break

Daily coffee break

17:30 → 19:30 Reception

Weekly reception

Thursday, 24 January

10:00 → 10:30 Effects of spin-orbit coupling on the BCS-BEC crossover

We study theoretically the effects of spin-orbit coupling on a two-spin-component ultracold atomic Fermi gas along the BCS-BEC crossover of a Feshbach resonance. We find that the condensate fraction of Cooper pairs characterizes the crossover better than other quantities, like the chemical potential or the pairing gap. We also find that, due to the spin-orbit coupling, in addition to singlet pairing, there is a finite triplet pairing. We predict that a large enough spin-orbit interaction enhances the singlet condensate fraction with respect to the triplet one in the BCS side while suppressing it on the BEC side. [1] L. Dell'Anna, G. Mazzarella, L. Salasnich, Phys. Rev. A 84, 033633 (2011). [2] L. Dell'Anna, G. Mazzarella, L. Salasnich, Phys. Rev. A 86, 053632 (2012).

Speaker: Prof. Luca Salasnich (Department of Physics, University of Padova)

10:30 → 11:00 BCS-BEC crossover in a quasi-2D Fermi gas

We consider a gas of fermionic atoms conned to a quasi- 2D geometry by a strong harmonic confinement potential in the transverse direction. For a two-component population balanced system, we construct a mean field theory for the BCS-BEC crossover, which correctly renormalises the s-wave contact interaction and allows infinitely many harmonic oscillator bands to be taken into account. The two band calculation can be done analytically giving the first order correction to the 2D results. However, we note that pairing is strongly modified by the presence of higher harmonic oscillator bands, even for weak interactions and Fermi energies much smaller than the confinement energy. We argue that recent experiments on pairing in quasi-2D Fermi gases [Y. Zhang et al., Phys. Rev. Lett. 108, 235302 (2012)] have already observed the expects of higher transverse levels.

Speaker: Dr Andrea Fischer (University of Cambridge, Cavendish Laboratory)

Slides Nordita_presentation_AFischer.pdf

14:00 → 14:45 Integrable pairing models in cold atom physics

The exact solution of the SU(2) pairing Hamiltonian with non-degenerate single particle orbits was introduced by Richardson in the early sixties, although it was recovered in the last decade in an effort to describe the disappearance of superconductivity in ultrasmall grains. Since then it has been widely applied to mesoscopic systems where finite size effects play an important role. Lately we have extended this family of exactly solvable models to higher rank algebras to describe pairing between multi-component fermion systems like three and four color atomic gases. In this talk I will review some of the achievements in the application of the exactly solvable Richardson models to cold atomic gases. I will start with the description of the BCS-BEC crossover view from the exact Richardson wavefunction which defines uniquely the structure of the Cooper pairs from extended resonances in the BCS regime to tightly bound molecules in the BEC regime. Another recent development is associated to the implementation of the hyperbolic family of the SU(2) Richardson model to describe p-wave pairing. Using this new tool we study the quantum phase diagram of a spinless Fermi gas in a 2D optical lattice with px + i py pairing interaction symmetry. Unlike the case of s-wave pairing, which has a smooth a crossover between BCS and BEC, p-wave pairing displays a quantum phase transition separating two gapped superfluid phases known as weak-pairing and strong-pairing. We use the exact solution as well as mean-field to characterize the quantum phase transition and the properties of the two phases. Finally, as an example of the higher rank Richardson models, I will introduce the exact solution of an SO(6) Richardson model that describes pairing between three component atomic fermions. The study of this new exactly solvable model allowed us to unveil a complex structure of breached pairing phases. We find two competing superfluid phases at weak and intermediate couplings, each with two-color pair condensates that can be distinguished with density profile measurements.

Speaker: Prof. Jorge Dukelsky (Instituto de Estructura de la Materia. CSIC.)

Slides Dukelsky-Nordita.pdf

15:00 → 16:00 Coffee break

Friday, 25 January

10:00 → 10:30 Three-body problem in a two-dimensional Fermi gas

We investigate the three-body properties of two identical "up" fermions and one distinguishable "down" atom interacting in a strongly confined two-dimensional geometry. We compute exactly the atom-dimer scattering properties and the three-body recombination rate as a function of collision energy and mass ratio m_up/m_down. We find that the recombination rate for fermions is strongly energy dependent, similarly to what was found for two-dimensional identical bosons. For m_up < m_down, the s-wave atom-dimer scattering below threshold is completely described by the scattering length. Furthermore, we examine the "up-up- down" bound states (trimers) appearing at large m_up/m_down and find that the energy spectrum for the deepest bound trimers resembles that of a hydrogen atom confined to two dimensions.

Speaker: Dr Jesper Levinsen (University of Cambridge)

10:30 → 11:00 Three-body states in a many-body background

The famous prediction of Efimov [1] that an infinitude of three-body bound states appear in shortrange interacting three-dimensional systems when there is a two-body bound state at zero energy has generated a large amount of interest in the cold atomic gas community after its initial observation in 133Cs [2]. The theoretical description of these experiments have thus far used the vacuum formalism. However, current experiments are in a regime where the background energy scale (such as the Fermi energy in degenerate Fermi systems) can play a significant role. We demonstrate that while Efimov states can be strongly perturbed by the background, the original scaling ideas play a crucial role for the manner in which the states change. In fact, we find that scaling laws related to the background parameters emerges that should be observable in current experiments [3]. References: [1] Efimov, V. Weakly bound states of three resonantly interacting particles. Yad. Fiz. 12, 1080-1091 (1970); Sov. J. Nucl. Phys. 12, 589-595 (1971); [2] T. Kraemer et al., Evidence for Efimov quantum states in an ultracold gas of caesium atoms, Nature 440, pp. 315-318 (2006). [3] N. G. Nygaard and N. T. Zinner, The Fate of the Efimov Effect in a Many-Body World, arXiv:1110.5854

Speaker: Prof. Nikolaj Zinner (University of Århus)

14:00 → 14:45 Suppression of the quantum-mechanical collapse by repulsive interactions

The quantum-mechanical collapse (alias "fall onto the center" of particles attracted by potential -1/r^2) is a well- known issue in the elementary quantum theory. It is closely related to the so-called "quantum anomaly", i.e., breaking of the scaling invariance of the respective Hamiltonian by the quantization. We demonstrate that, in a rarefied gas of quantum particles attracted by the above- mentioned potential, the mean-field repulsive nonlinearity induced by collisions between the particles prevents the collapse, and thus puts forward a solution to the quantum- anomaly problem different from that previously developed in the framework of the linear quantum-field theory. This solution may be realized in the 3D or 2D gas of dipolar bosons attracted by a central charge, and also in the 2D gas of magnetic dipoles attracted by a current filament. In lieu of the collapse, the cubic nonlinearity creates a 3D ground state (GS), which does not exist in the respective linear Schroedinger equation. The addition of the harmonic trapping potential gives rise to a tristability, in the case when the Schroedinger equation still does not lead to the collapse. In the 2D setting, the cubic nonlinearity is not strong enough to prevent the collapse; however, the quintic term does it. The analysis is also extended to the 3D anisotropic setting, with the dipoles polarized by an external uniform field. Publications: H. Sakaguchi and B. A. Malomed, Suppression of the quantum-mechanical collapse by repulsive interactions in a quantum gas, Phys. Rev. A 83, 013907 (2011); H. Sakaguchi and B. A. Malomed, Suppression of the quantum collapse in an anisotropic gas of dipolar bosons, Phys. Rev. A 84, 033616 (2011).

Speaker: Prof. Boris Malomed (Tel Aviv University, Faculty of Engineering, Dept. of Physical Electronics)

Slides B_Malomed.ppt

15:00 → 16:00 Coffee break

Monday, 28 January

10:00 → 10:45 The SU(N) Heisenberg model of ultracold fermionic alkaline earth atoms

In this talk, I will review the recent results we have obtained on the SU(N) Heisenberg model of the Mott insulating phase of multi-color ultracold fermionic atoms loaded in optical lattices with various geometries. In 1D, where Quantum Monte Carlo simulations can be performed, we have calculated the correlations as a function of the entropy per site, with the conclusion that the entropy below which characteristic features show up increases with N and reaches experimentally accessible values already for N=4 [1]. In 2D, using a variety of analytical and numerical approaches, in particular flavour-wave theory and a tensor-network algorithm, we have shown that the nature of the ground state depends crucially on the value of N and on the topology of the lattice, including long-range color order for SU(3) on the triangular and square lattices [2], spontaneous dimerization for SU(4) on the square lattice [3], and an algebraic quantum liquid for SU(4) on the honeycomb lattice [4]. Experimental implications for fermionic alkaline earth atoms will be briefly discussed. [1] L. Messio and F. Mila, PRL 109, 205306 (2012). [2] T. Toth, A. Laeuchli, F. Mila, K. Penc, PRL 105, 265301 (2010). [3] P. Corboz, A. Laeuchli, K. Penc, M. Troyer, F. Mila, PRL 107, 215301 (2011). [4] P. Corboz, M. Lajko, A. Laeuchli, K. Penc, F. Mila, PRX 2, 041013 (2012)

Speaker: Prof. Frederic Mila (Ecole Polytechnique Federale de Lausanne)

14:00 → 14:45 Novel scenarios for atoms in optical lattices

Atoms in optical lattices present exciting possibilities of control and quantum engineering. In this talk I would like to discuss two different scenarios which may be attained within the current state of the art. I will first discuss the case of bosons in zig-zag optical lattices [1], which may be created using superlattice techniques. For the case of unconstrained bosons I will discuss in particular chiral phases and the possibility of observing Mott-phases at vanishingly low interactions. I will also discuss the case of bosons with a two-body hard-core constraint, which becomes possible for large three-body losses. I will show that in that case the system presents a rich phase diagram, which includes Haldane-insulator, pair-superfluid, density-wave and chiral-superfluid phases. In the second part of the talk I will discuss atoms in optical lattices with periodically-modulated interactions. This periodic modulation results in an effective nonlinear hopping which leads to interesting phases, including pair- superfluids, defect-free Mott insulators, and holon- and doublon-superfluids [2]. [1] S. Greschner, L. Santos and T. Vekua, arXiv:1202.5386 [2] A. Rapp, X. Deng and L. Santos, Phys. Rev. Lett. 109, 203005 (2012).

Speaker: Prof. Luis Santos (Leibniz Universität Hannover)

15:00 → 16:00 Coffee break

Tuesday, 29 January

10:00 → 10:45 Dipolar Bose-Einstein condensates in 1D lattices - dipolar stabilization, de-stabilization and collapse

In my talk I will try to to shine a light on the peculiar stability conditions of dipolar Bose-Einstein condensates trapped in 1D optical lattices. All effects connected with the formation of ordered states in dipolar quantum gases appear close to the border between stability and instability of the trapped gas. These features are mediated by the interplay between short-range and long-range, isotropic and anisotropic interactions and the trap. Due to the anisotropy of dipole-dipole interaction, the stability properties are in fact strongly affected by the confining potential. We have experimentally studied these stability conditions for various scenarios with a dipolar 52Cr Bose-Einstein condensate, especially in the relevant case of periodic 1D lattice potentials where we observe that inter-sites effects play an important role. A 1D lattice, in contrast to a purely contact interacting gas, induces a crossover from a dipolar de-stabilization to a dipolar stabilized regime with increasing lattice depth. In a deep lattice, a dipolar condensate can be stabilized even at large negative scattering length in the interaction-dominated regime. As an important consequence, a dipolar condensate can be stable in trap but immediately collapse as soon as the external confinement is removed. This characteristic feature of strongly dipolar BECs makes the usual mapping from time-of flight measurements on the momentum distribution highly non-trivial.

Speaker: Prof. Axel Griesmaier (University of Stuttgardt, Germany)

14:00 → 14:45 Ultracold atoms in 2D optical lattices as cond-mat emulators

During the last years, cold atoms loaded into optical lattices emerged as an ideal playground to emulate condensed matter systems. In this talk, I will first discuss a recently proposed experimental set-up, which allows for the realization of a spin-dependent optical lattice, in which an effective Zeeman coupling can be generated by Raman excitations [1]. The model Hamiltonian is quite simple, but leads to very rich physics: the ground state bears similarities with a coupled spin- and charge-density wave state. A path integral formalism is discussed, based on a Hubbard-Stratonovich transformation, which allows one to treat both, spin and charge within RPA, on the same footing. In the second part, I will discuss a 3-band Hubbard model in a bipartite 2D optical lattice, to describe recent experiments in which an interaction induced $p_x + i p_y$ BEC is stabilized [2]. [1] D. Makogon, I. B. Spielman, and C. Morais Smith, EPL 97, 33002 (2012); Editor choice EPL 2012. [2] M. Olschlager, T. Kock, G. Wirth, A. Ewerbeck, C. Morais Smith, and A. Hemmerich, preprint 2012.

Speaker: Prof. Cristiane Morais Smith (Institute for Theoretical Physics, Utrecht)

15:00 → 16:00 Coffee break

Wednesday, 30 January

11:00 → 11:45 Dynamical quantum simulation with ultra-cold atoms

Quantum simulators promise to simulate the dynamics of complex quantum systems in a more efficient way than what is classically possible. In this talk, we will elaborate on the question in what sense experiments with ultra-cold atoms in optical lattices can fulfil the promise of being first dynamical quantum simulators truly outperforming classical devices. We will discuss the potential of such systems probing questions of equilibration and thermalisation, questions that are otherwise hard to assess, both theoretically and numerically. In the last part, I will argue that the sampling problem "solved" by the quantum experiment falls within a class of problems that is classically intractable, by relating it to the boson sampling problem and the polynomial hierarchy of complexity classes in computer science. [1] S. Trotzky, Y.-A. Chen, A. Flesch, I. P. McCulloch, U. Schollwoeck, J. Eisert, I. Bloch, Nature Physics 8, 325 (2012). [2] M. Cramer, A. Flesch, I. P. McCulloch, U. Schollwoeck, J. Eisert, Phys. Rev. Lett. 101, 063001 (2008). [3] J. Eisert, M. Kliesch, in preparation (2012).

Speaker: Prof. Jens Eisert (Freie Universität Berlin)

14:00 → 14:45 Dynamically creating artificial gauge potentials in optical lattices

In the last decade there has been considerable progress in the experimental realization of artificial many-body systems made of ultracold neutral atoms in optical lattices potentials. These systems are extremely clean, well isolated from their environment, and highly tunable (also during the experiment). This makes them a flexible platform for engineering many-body quantum physics in and also out-of equilibirum. An important ingredient is the abilty to create artificial gauge potentials that allow to mimic strong magnetic fields. Here one aim is to realize quantum Hall- type physics in the regime where the length scale of the lattice matters, like in the strong-field regime (captured by the Harper model) or like in topological insualtors were appropriately chosen staggered fields lead to a quantized Hall conductivity for a completely filled band. Pioneering experiments in which artificial gauge potentials have been created in optical lattices have been reported recently by Aidelsburger et al. (PRL 2011), Jimenez-Garcia et al. (PRL 2012), and Struck et al. (Science 2011, PRL 2012). I will talk about the theory behind the approach of Struck et al. where a gauge potential is induced dynamically by fast lattice shaking [see also Eckardt et al. EPL 2010, Hauke et al. PRL 2012]. The shaken lattice is a Floquet-system and its dynamics is captured by an effective time-independent Hamiltonian that is obtained by integrating out the rapid dynamics within a period of the forcing. Temporal symmetries are indientified that have to be broken in order to achieve tunable gauge potentials. I will also discuss applications of this method, for example how it can be used to realize a topological insulator and how it can be generalized to create non-abelian gauge fields in spin- dependent lattices.

Speaker: Dr Andre Eckardt (MPIPKS Dresden)

15:00 → 15:15 Coffee break

17:30 → 19:30 Reception

Weekly reception

Thursday, 31 January

10:00 → 10:30 Interaction-induced Anderson localisation of atoms in a bichromatic optical lattice

We investigate the formation of bound states made of two interacting atoms moving in a one dimensional quasi-periodic optical lattice. We derive the quantum phase diagram for Anderson localization of both attractively and repulsively bound pairs. We calculate the pair binding energy and show analytically that its behavior as a function of the interaction strength depends crucially on the nature -extended, multifractal, localized- of the single-particle atomic states. Experimental implications of our results are discussed. Ref: Phys. Rev. Lett. 109, 155306 (2012)

Speaker: Prof. Giuliano Orso (Université Paris Diderot)

14:00 → 14:45 Unconventional superfluidity in higher bands of an optical lattice

Atoms trapped in optical lattices hold promises as a useful new arena for studying many-body phenomena, possibly providing helpful insights with regard to various incompletely understood condensed matter scenarios. Unfortunately, the shape that bosonic ground-state wavefunctions can take is limited, apparently compromising the usefulness of this approach for bosons. Such limitations, however, do not apply to excited states, where orbital degrees of freedom are essential. I will discuss our observations of long coherence times, chiral superfluid order and topological features in higher bands in a square optical lattice.

Speaker: Prof. Andreas Hemmerich (Hamburg University)

15:00 → 16:00 Coffee break

Friday, 1 February

10:00 → 10:30 Zero-energy (Majorana) quasiparticles in fermionic superfluids in 2D optical lattices

A class of gapped many-body systems displays zero- energy (Majorana) quasiparticles with non-Abelian statistics and as a consequence possesses peculiar topological phases [1, 2]. We discuss here how these arise naturally in fermionic superfluids in 2D optical lattices, in two different scenarios which may be soon realized experimentally. In first instance, we show how to create a stable p- wave superfluid using bosons mixed with a single species of fermions [3]. We analyze with a self- consistent method its excitation spectrum in presence of a vortex, and we point out the most favorable range of interaction strengths in which the zero-energy mode with topological character may be observed on a finite optical lattice. We also discuss how topological phases appear in imbalanced two-component Fermi superfluids with standard s-wave interactions, when the gas is exposed to a synthetic non-Abelian magnetic field [4, 5]. We present the complete topological phase diagram, and we analyze the connection between Chern numbers and the existence of topologically protected edge modes. Moreover, we address the issue of superfluidity breakdown by spin imbalance, indicating which of the regions in the phase diagram are experimentally accessible. #### #### #### #### References [1] C. Nayak, S. Simon, A. Stern, M. Freedman, and S. Das Sarma, Rev. Mod. Phys. 80, 1083 (2008). [2] N. Read and D. Green, Phys. Rev. B 61, 10267 (2000). [3] P. Massignan, A. Sanpera, and M. Lewenstein, Phys. Rev. A 81, 031607(R) (2010). [4] M. Sato, Y. Takahashi, and S. Fujimoto, Phys. Rev. Lett. 103, 020401 (2009). [5] A. Kubasiak, P. Massignan, and M. Lewenstein, Europhys. Lett. 92, 46004 (2011)

Speaker: Dr Pietro Massignan (ICFO - Institute of Photonic Sciences)

Slides Massignan_Nordita_TopSF.pdf

10:30 → 11:00 Topological states of matter and their perspectives in cold atom systems

Topological states of matter which can be understood at the level of quadratic Hamiltonians have been in the spotlight of condensed matter physics in recent years and also the cold atoms community is currently developing a major focus on this topic. In this talk, the fundamental notion of topological states of matter is reviewed in an accessible way. Furthermore, some very recent developments are discussed which open new perspectives that are peculiar to cold atom systems where both interactions and quantum dissipation can be engineered.

Speaker: Dr Jan Budich (Stockholm University)

14:00 → 14:30 Ultracold bosons in double- and triple-well potentials

We consider ultracold and dilute bosonic atoms confined by double-well shaped potentials. By employing the two- site Bose-Hubbard (BH) model as a theoretical tool, we describe the behaviour of such a system both at zero and at finite temperature. The ground-state of the two-site BH Hamiltonian will be studied by analyzing how the inter-atomic interaction affects the quantum Fisher information, the coherence visibility, and the entanglement entropy, by focusing, in particular, on the emergence of the Schrödinger's cat like state. In the presence of the temperature, I will show that, contrary to naive expectations, when the boson-boson interaction is suitably chosen thermal effects can increase the coherence visibility. Finally, we consider ultracold dipolar bosonic atoms trapped by triple-well potentials in the presence of periodic boundary conditions. By diagonalizing the three-mode extended BH Hamiltonian, we study the ground-state of the system by characterizing it by means of the entanglement entropy.

Speaker: Dr Giovanni Mazzarella (Università di Padova)

14:30 → 15:00 Dissipation-driven squeezing

Dissipation is typically considered to be a serious enemy to quantum systems as it leads to a rapid decay of the coherence. Surprisingly, however, recent studies show that an appropriately designed coupling between the system and the reservoir can drive the system into a given pure state [1,2]. This opens the way for the use of dissipation in quantum state engineering. Here we present a method to create phase- and number- squeezed states in two-mode Bose systems using dissipation [3]. Creating squeezed states is a key issue in interferometry as they allow the improvement of precision measurements beyond the conventional bound attainable by classical means. The effectiveness of this method is demonstrated by considering cold Bose gases trapped in a double-well potential. The extension of our formalism to an optical lattice gives control of the phase boundaries of the steady-state phase diagram, and we discover a new phase characterized by a non-zero condensate fraction and thermal-like particle number statistics. We also propose a physical setup to realize our sceme. References [1] S. Diehl et al., Nature Phys. 4, 878 (2008). [2] B. Kraus et al., Phys. Rev. A 78, 042307 (2008). [3] G. Watanabe and H. Mäkelä, Phys. Rev. A 85, 023604 (2012).

Speaker: Prof. Gentaro Watanabe (Asia Pacific Center for Theoretical Physics (APCTP))

Slides dissipation_gentaro_watanabe.pdf

15:00 → 16:00 Coffee break

Monday, 4 February

10:00 → 10:45 Topological phases of fermions in the p-orbital band of optical lattices

An exciting thrust of cold atom research is to explore some unique aspects of such systems that have no prior analogue in electronic solids. An emergent topic along this line is the study of cold atoms coherently excited up to the higher orbital bands of optical lattices in recent experiments, motivated in part by early theoretical proposals. In this talk, I will report in theory that such orbital systems can show intriguing phases of matter. For example, interacting fermions on a two-leg ladder of unequal parity orbitals, which is derived from the experimentally realized double- well lattices by dimension reduction, are found topological in calculation. References: [1] Nat. Phys. 7, 101 (2011); [2] Nat. Phys. 8, 6770 (2012); [3] arXiv:1205.0254. Work done in collaboration with S. Das Sarma, A. Hemmerich, M. Lewenstein, X. Li, K. Sun, and E. Zhao. Acknowledge support by ARO, AFOSR, and DARPA of the U.S. DOD, A. Mellon Foundation, and NSF of China.

Speaker: Prof. W. Vincent Liu (University of Pittsburgh)

13:00 → 13:45 Quantum magnetism of ultracold fermions in an optical lattice

We report on the observation of short-range quantum magnetic correlations of fermionic atoms in an optical lattice. The key to entering the regime of quantum magnetism is a tunable geometry optical lattice, which allows us to locally redistribute the entropy. When loading a low-temperature two-component gas with repulsive interactions into either a dimerized or anisotropic simple cubic lattice, we find magnetic correlations on neighbouring sites. The correlations manifest as an excess number of singlets as compared to triplets consisting of two atoms with opposite spins. For the anisotropic lattice, we determine the transverse spin correlator from the singlet-triplet imbalance and observe antiferromagnetic correlations along one spatial axis.

Speaker: Prof. Tilman Esslinger (ETH, Zürich)

14:00 → 14:30 Coffee break

15:00 → 16:00 New Frontiers in Quantum Simulation with Cold Atoms and Ions: Prospects of Simulating Lattice Gauge Theories

Recently, the condensed matter and atomic physics communities have mutually benefited from synergies emerging from the quantum simulation of strongly correlated systems using atomic setups. In the first part of the talk we give an overview of analog and digital quantum simulation with cold atoms in optical lattices and trapped ions. In the second part we discuss possible future directions: while there is presently significant interest in artificial gauge fields mimicking magnetic fields in (neutral) atom setups to observe phenomena like fractional quantum Hall physics, we will discuss prospects of realizing simple models of dynamical gauge fields (lattice gauge theories) as a next generation of possible cold atom experiments, where the (very) long term goal is to develop atomic quantum simulators for QED and QCD.

Speaker: Prof. Peter Zoller (University of Insbruck)

16:20 → 16:50 From p-orbital bosons to simulation of anti-ferromagnetic Heisenberg spin models

TBA

Speaker: Ms Fernanda Pinheiro (Stockholm University/NORDITA)

16:50 → 17:20 Exploring spin-orbital models with cold gases loaded in the p-bands of optical lattices

We will discuss possibility of simulating exotic phases of orbitally degenerate frustrated antiferomagnets in cold gases. Two component Fermi gas loaded in the p-bands of the zig-zag type optical lattice can mimic solid state spin-orbital models such as transition-metal oxides with partially filled d-levels, with the interesting advantage of reviving the quantum nature of orbital fluctuations. As a result of interplay between lattice geometry and orbital degeneracy number of unusual phases can be realized, including those with topological order. The talk will be based on 2 works, one published in: G. Sun, G. Jackeli, L. Santos, and T. Vekua, Phys. Rev. B 86, 155159 (2012).

Speaker: Dr Teimuraz Vekua (Leibniz University of Hannover)

Tuesday, 5 February

10:00 → 10:45 Quantum gases of ultracold polar molecules

TBA

Speaker: Prof. Silke Ospelkaus (Leibniz University Hannover)

14:00 → 14:30 Manifestation of spin-orbit coupling for ultra-cold atoms

Over the last several years there has been a substantial increase of interest in artificial gauge fields and spin-orbit coupling for electrically neutral atoms [1–3]. The spin-orbit coupling with equal Rashba and Dresselhaus contributions has been recently implemented experimentally [4]. Here we consider manifestations of such a spin-orbit coupling for scattering of atoms at the impurity sites [5]. In particular, we show that the spin-orbit coupling can lead to both suppression or enhancement of the atomic backward scattering. Another topic to be discussed is creation of the artificial magnetic field involving synthetic dimensions associated with the atomic internal degrees of freedom [6]. [1] M. Lewenstein, A. Sanpera, V. Ahufinger, B. Damski, A. S. De, and U. Sen, Adv. Phys. 56, 243 (2007). [2] I. Bloch, J. Dalibard, and W. Zwerger, Rev. Mod. Phys 80, 885 (2008). [3] J. Dalibard, F. Gerbier, G. Juzeliūnas, and P. Öhberg, Rev. Mod. Phys 83, 1523 (2011). [4] Y.-J. Lin1, K. Jimenez-Garcıa and I. B. Spielman, Nature(London) 471, 83 (2011). [5] J. Ruseckas, R. Juršėnas, G. Juzeliūnas and I.B. Spielman, in preparation. [6] G. Juzeliūnas, I.B. Spielman, A. Celi, P. Massignan and M. Lewenstein, in preparation.

Speaker: Prof. Gediminas Juzeliunas (Vilnius University)

14:30 → 15:00 Quantum Quench of a p-Wave Fermi Gas

I will talk about an ongoing project on the non-equilibrium dynamics following a quantum quench in a p-wave superfluid Fermi gas at zero temperature. P- wave superfluid is known to undergo a quantum phase transition from a gapless phase to a gapped phase when the p-wave interaction is tuned from BCS to BEC regime. The dynamics of a polar state (possibly, axial state in the near future) in the p-wave superfluid Fermi gas after a sudden quench of the p-wave interaction is studied within a mean field approach. The time evolution of the order parameter is obtained by solving the BdG equations numerically and its behavior is compared with that of the s-wave case where the change from the BCS to BEC regime is just a crossover.

Speaker: Dr Sukjin Yoon (Asia Pacific Center for Theoretical Physics)

15:00 → 15:30 Coffee break

15:30 → 16:00 Bloch oscillatio​ns of cold atoms in cavities

Quantum particles in a periodic potential subject to an additional linear force undergo Bloch oscillation at a frequency that is directly proportional to the magnitude of the applied force. In this talk I will describe a novel in- situ, non-destructive method to measure the Bloch frequency for a cloud of cold atoms that are confined within the electromagnetic field of a high quality standing wave cavity. The idea is to use the classical back-action of the atoms on the intra-cavity light field that contains signatures of the atomic dynamics (the Bloch frequency) and measure the amplitude and/or phase of the transmitted light field. As with any cavity based continuous measurement scheme we also need to consider the effects of quantum measurement back- action on this proposal. We do this by considering the dynamics of quantised linearised fluctuations about the dynamical classical atomic and light fields.

Speaker: Mr Prasanna Venkatesh (McMaster University)

Wednesday, 6 February

10:00 → 10:45 Artificial gauge potentials with flux lattices

Among the large variety of quantum collective phenomena that one hopes to address with atomic vapours, magnetism is one of the richest. However the quest for the simulation of magnetism immediately raises a challenging question: how can a system of neutral atoms behave as an assembly of charged particles in a magnetic field? The talk will review some promising approaches to answer this question, focusing in particular on the concept of “flux lattices”. I will explain the principles at the basis of these lattices, which allow one to reach with a quasi-flat lowest band with non- trivial topological properties. I will also discuss why they are well suited to produce atomic states that would be analogous to fractional quantum Hall states.

Speaker: Prof. Jean Dalibard (Collège de France et Ecole normale supérieure)

14:00 → 14:45 Information thermodynamics and fluctuation theorem

The second law of thermodynamics presupposes a clear-cut distinction between the controllable and uncontrollable degrees of freedom by means of macroscopic operations. The cutting-edge technologies in quantum information and nanoscience seem to require us to abondon such a working hypothesis in favor of the distinction between the accessible and inaccessible degrees of freedom. In this talk, I will talk about the fundamentals of such information thermodynamics together with the related new results on fluctuation theorems.

Speaker: Prof. Masahito Ueda (University of Tokyo)

15:00 → 15:30 Coffee break

15:30 → 16:00 Simulating an interacting gauge theory with ultracold Bose gases

We show how density dependent gauge potentials can be induced in dilute gases of ultracold atoms using light-matter interactions. We study the effect of the resulting interacting gauge theory and show how it gives rise to novel topological states in the ultracold gas. We find in particular that the onset of persistent currents in a ring geometry is governed by a critical number of particles. The density-dependent gauge potential is also found to support chiral solitons in a quasi-one-dimensional ultracold Bose gas.

Speaker: Dr Öhberg Patrik (Heriot-Watt University)

17:30 → 19:30 Reception

Weekly reception

Thursday, 7 February

10:00 → 10:30 The fate of Bose-Einstein condensate in the presence of spin-orbit coupling

The recent realization of synthetic gauge fields for ultra cold atoms provides physicists exciting opportunities to investigate the interplay between two fundamental phenomena in nature, Bose-Einstein condensation and spin-orbit coupling. In this talk, I will discuss a novel effect of spin-orbit coupling in bosonic systems, namely, it can destroy a high-dimensional condensate even at sufficiently low temperatures. This effect will be first demonstrated using simple examples of non-interacting bosons, which highlight the underlying physics that spin-orbit coupling qualitatively changes the single-particle Density of States at low energies. I will then turn to interacting systems, where a condensate is stabilized by interaction at zero temperature. On the other hand, condensate depletion is significantly enhanced by spin-orbit coupling. Particularly, thermal depletion becomes divergent when spin-orbit coupling becomes isotropic and interaction is spin-independent. This leads to the disappearance of a three-dimensional condensate at any finite temperature, and suggests an interesting routine to suppress the long-range order in weakly interacting atomic systems via spin-orbit coupling. I will also briefly discuss how spin-orbit coupling may fundamentally change the properties of low-dimensional bosons.

Speaker: Prof. Qi Zhou (The Chinese University of Hong Kong)

10:30 → 11:00 'Canonical' Majorana fermions and emergent gauge theories in ultracold gases

Motivated by ground-breaking experimental findings, static gauge potentials and topological phases of matter are currently two of the most intriguing topics in cold atom physics. In the first part of the talk, we will show how topological phases supporting Majorana edge states emerge in simple toy models of fermionic ladders without the need of any additional reservoir by exploiting the mutual symmetries of ladder setups. We will then discuss how a key ingredient of these toy models, namely inter-wire pair tunneling, can be realized in cold gases in optical lattices. In the second part of the talk, we will show how the physics of dynamical gauge fields, which play a key-role in the Standard model of particle physics and in frustrated spin systems, emerges in multi-component Bose and Fermi mixtures, providing a route toward the quantum simulation of confinement phenomena.

Speaker: Dr Marcello Dalmonte (Insbruck)

14:00 → 14:45 Bloch, Immanuel: TBA

TBA

15:00 → 15:30 Coffee break

Friday, 8 February

10:00 → 10:45 Quantum chaos and effective thermalization

We will discuss mechanisms of effective equilibration ('thermalization') for unitary quantum dynamics under conditions of classical chaos. Focusing on the paradigmatic example of the Dicke model, we will explore how a constructive description of the thermalization process is facilitated by the Glauber Q or Husimi function, for which the evolution equation turns out to be of Fokker-Planck type. The equation describes a competition of classical drift and quantum diffusion in contractive and expansive directions. By this mechanism the system follows a ’quantum smoothened’ approach to equilibrium, which avoids the notorious singularities inherent to classical chaotic flows. Other universal signatures of the equilibration of nonlinear quantum systems will be discussed on the toy model of a nonlinear ocillator mode.

Speaker: Prof. Alexander Altland (Köln Universität)

14:00 → 14:45 Magnetism without magnetism

TBA

Speaker: Prof. Klaus Sengstock (University of Hamburg)

15:00 → 15:30 Coffee break

15:30 → 16:00 Hall conductivity in spin-orbit coupled bosonic Mott insulator

We study the Hall conductivity in a spin-orbit coupled bosonic Mott insulator. Using a strong-coupling perturbation theory, we show that in the spinful Bose Hubbard model, interactions can induce momentum-space Berry curvature, leading to the anomalous Hall phase. Furthermore, we find that the ground state can in principle support an integer Hall conductivity, i.e., the quantum anomalous Hall phase, if the sum of the Chern numbers of the hole bands is nonzero. For the abelian spin-orbit coupled Bose gas that has been achieved in cold atom experiments, we propose some experimental signatures of the interaction induced Berry curvature. Our results have implications for topological transport in the Mott-insulating phases with textured magnetic order.

Speaker: Dr Clement Wong (Utrecht University)

Slides boseMotttalk.key

Monday, 11 February

10:00 → 10:45 Efimov Physics beyond universality, Parity-order in Mott-Insulators, Duality, and Gauge Fields

Speaker: Prof. Wilhelm Zwerger (Technische Universität München)

Slides Zwerger_Nordita.pdf

14:00 → 14:45 Ultracold atoms in Florence: disorder and new experiments

TBA

Speaker: Prof. Massimo Inguscio (University of Florence)

15:00 → 15:30 Coffee break

15:30 → 16:00 Bosons with Synthetic Rashba Spin-Orbit Coupling at Finite Power

Isotropic spin-orbit couplings, such as Rashba in two dimensions, have a continuous symmetry that produces a large degeneracy in the momentum-space dispersion. This degeneracy leads to an enhanced density-of-states, producing novel phases in systems of bosonic atoms. This model is idealistic, however, in that the symmetry of the lasers will weakly break the continuous symmetry to a discrete one in experimental manifestations. This perturbation typically scales inversely with the optical power, and only at infinite power will ideal symmetry be restored. In this talk, we consider the effects of this weak symmetry breaking in a system of bosons at finite power with synthetic Rashba coupling. We solve the mean-field equations and find new phases, such as a stripe phase with a larger symmetry group. We then consider the experimentally relevant scheme where the spin-orbit fields are turned on adiabatically from an initial spin-polarized state. At intermediate power, stripe phases are found, while at sufficiently high power it appears that the system quenches to phases similar to that of the ideal limit. Techniques for optimizing the adiabatic ramping sequence are discussed.

Speaker: Dr Brandon Anderson (Joint Quantum Institute)

16:00 → 16:30 Phase diagram of quasi-two-dimensional bosons in laser speckle potential

We have studied the phase diagram of a quasi-two-dimensional interacting Bose gas at zero temperature in the presence of random potential created by laser speckles. The superfluid fraction and the fraction of particles with zero momentum are obtained within the mean-field Gross-Pitaevskii theory and in diffusion Monte Carlo simulations. We find a smooth crossover from the superfluid to the insulating state, when the strength of the disorder grows. Estimations of the crossover lines are compared with the predictions of the percolation theory in the Thomas-Fermi approximation. Analytical expressions for zero-momentum fraction and superfluid fraction are derived in the limit of weak disorder and weak interactions within the framework of the Bogoliubov theory. Limits of validity of various approximations are discussed.

Speaker: Dr Konstantin Krutitsky (Universität Duisburg-Essen)

Tuesday, 12 February

10:00 → 10:30 Fermi-liquid theory of imbalanced quark matter

The temperature dependence of the thermodynamic potential of quantum chromodynamics (QCD), the specific heat, and the quark effective mass are calculated for imbalanced quark matter in the limit of a large number of quark flavors (large-NF), which corresponds to the random-phase approximation. Also a generalization of the relativistic Landau effective-mass relation in the imbalanced case is given, which is then applied to this thermodynamic potential.

Speaker: Dr Shaoyu Yin (Department of Applied Physics, Aalto University)

10:30 → 11:00 Mean-field description of a rotating dipolar Bose gas in an annular trap and of a rapidly rotating two-component Bose gas

The high degree of tunability and flexible control are the two important features associated with cold atomic gases, which have so far paved the way for various applications including ow-dimensional systems, confining potentials with different functional forms, multi-component Bose-Einstein condensates (BECs), quantum gases with different inter-particle interactions, etc. Systems with one or more of these properties exhibit remarkably rich physics, which is interesting to study either experimentally or theoretically. A good example to see the effect of such properties coexisting in a system is a low-dimensional dipolar Bose gas. Low-dimensional confinements offer the opportunity to study the effects of dipolar interaction without instability problems caused by the head-to-tail alignment of dipoles in three dimensions. To investigate the anisotropic character of the interaction, we consider a rotating dipolar BEC confined in an annular trap for an arbitrary orientation of the dipoles with respect to their plane of motion. Within the mean-field approximation, we find that the system exhibits different vortex configurations depending on the polarization angle of the dipoles and on the relative strength between the dipolar and the contact interactions. Another example of such a system is a two-component BEC confined in an anharmonic potential. Confining potentials rising more steeply than quadratically allow the study of rapidly rotating BECs, which introduce many novel phases. This picture becomes even more interesting in the case of a multi-component BEC . We investigate the rotational properties of a two-component BEC, which is confined in an anharmonic trapping potential using both numerical and analytic methods. More specifically, with the use of a variational approach we derive analytically the phase diagram of the system as a function of the rotational frequency of the trap and of the coupling constant for sufficiently weak values of the anharmonicity and of the coupling. The more general structure of the phase diagram is investigated numerically. We compare our results with the ones of (i) a single-component BEC confined in an anharmonic potential, and (ii) a two-component BEC, which is confined in a harmonic trapping potential.

Speaker: Dr Elife Karabulut (Lund University)

14:00 → 14:45 Quantum-dot analogues with cold atoms – dipolar interactions, spin-orbit coupling and quantum transport

Cold atom systems offer many possibilities to shape mesoscopic quantum systems with properties that are fundamentally different from semiconductor nanostructures, such as quantum dots and quantum wires with electrons. The talk will provide a review on the many-body physics of these finite-size bosonic or fermionic quantum systems, with focus on the configuration interaction method. For bosonic systems, the relation between finite-size systems and the thermodynamic limit will be addressed [1]. For dipolar interactions in low-dimensional systems, the Wigner localization emerging with increasing coupling strength of the dipoles exhibits nontrivial geometries due to the anisotropy of the interaction [2]. In the regime of weaker interactions, for fermions with aligned dipole moments strong shell structure occurs, which is strongly diminished by changing the dipolar tilt angle [3]. The talk will also address the effect of Rashba-type spin-orbit coupling in the few-body limit [4]. Inspired by the recent experimental developments with atom transport [5,6] we furthermore investigate the analog of a quantum wire using ultra-cold particles, finding a new scenario for the quantum transport [7]: Attractive interactions may lead to a complete suppression of current in the low-bias range, a total current blockade. In particular, we consider this effect for the example of ultra-cold quantum gases with dipolar interactions. [1] J. Cremon, G.M. Kavoulakis, B.R. Mottelson and S.M. Reimann, submitted (2012). [2] J.Cremon, G.M. Bruun and S.M. Reimann, Phys. Rev. Lett. 105, 255301 (2010); S. Zöllner, G.M. Bruun, S.M. Reimann and C.J. Pethick, Phys. Rev. Lett. 107, 035301 (2011). [3] G. Eriksson, J. Cremon and S.M. Reimann, to be published. [4] A. Cavalli, F. Malet, J. Cremon and S.M. Reimann, Phys. Rev. B 84, 235117 (2011); Y. Yusefi et al., to be published. [5] J.P. Brantut, J. Meineke, D. Stadler, S. Krinner, and T. Esslinger, Science 337, 1069-1071 (2012) [6] D. Stadler, S. Krinner, J. Meineke, J.P. Brantut, and T. Esslinger, Nature 491, 736 (2012) [7] L.H. Kristinsdottir, O. Karlström, J. Bjerlin, J.C. Cremon, P. Schlagheck, A. Wacker, S.M. Reimann, to be published.

Speaker: Prof. Stephanie M Reimann (Lund University)

15:00 → 15:30 Coffee break

16:00 → 16:30 Beyond standard Hubbard models

A natural description of ultracold gases trapped in optical lattices is given by the so-called single-band Hubbard model. We will discuss the additional effects of interaction to induce intra- as well as inter-band scattering to neighbouring sites, which shows up as density dependent correlated tunneling processes. Such processes can spontaneously give rise to various exotic phases. As examples, we will present 1) spontaneous creation of non-Fermi liquid smectic metal phases in dipolar Fermions, and 2) dynamical generation of exotic lattices in Fermi-Fermi mixtures.

Speaker: Dr Omjyoti Dutta (ICFO - The Institute of Photonic Science)

Wednesday, 13 February

10:00 → 10:45 Spielman, Ian: TBA

TBA

14:00 → 14:45 Topological transitions in mixed-geometry lattices, and dynamics of fermions in one dimension

We propose a mixed-geometry system of fermionic species selectively confined in lattices of different geometry [1]. We investigate how such asymmetry can lead to exotic multiband fermion pairing in an example system of honeycomb and triangular lattices. A rich phase diagram of interband pairing with gapped and gapless excitations is found at zero temperature. We find that the two-band contribution of the honeycomb lattices to the paired state helps to stabilize the gapless phase with one or two Fermi surfaces. We also show that the Fermi surface topology further divides the gapless phase into subclasses between which the system undergoes density-driven Lifshitz transitions. We consider examples of interesting quantum dynamics of interacting fermions in a one-dimensional system (tube) with a lattice potential along the tube direction. We have applied the time-dependent DMRG method, and the Bethe ansatz in the strong interaction regime, to exactly simulate and qualitatively describe the dynamics. In particular, we characterize the expansion of a band-insulator state in and show how it can be understood in terms of two-site physics [2]. By similar studies, we have shown that the Fulde-Ferrell-Larkin- Ovchinnikov (FFLO) state is directly manifested in the expansion dynamics [3]. We have studied the stability of the FFLO state in the 1D-3D crossover [4]. We also discuss dynamics of an impurity in a one-dimensional Fermi gas which reveals a rich interplay of polaronic- type and bound-pair effects [5]. [1] D.-H. Kim, J.S.J. Lehikoinen, and P. Törmä, Topological transitions of gapless paired states in mixed-geometry lattices, accepted to Phys. Rev. Lett. (2013), arXiv:1209.1571. [2] J. Kajala, F. Massel, and P. Törmä, Expansion dynamics in the one-dimensional Fermi-Hubbard model, Phys. Rev. Lett. 106, 206401 (2011). [3] J. Kajala, F. Massel, and P. Törmä, Expansion dynamics of the Fulde-Ferrell-Larkin-Ovchinnikov state, Phys. Rev. A 84, 041601(R) (2011). [4] D.-H. Kim and P. Törmä, Fulde-Ferrell-Larkin- Ovchinnikov state in the dimensional crossover between one- and three-dimensional lattices, Phys. Rev. B 85, 180508(R) (2012). [5] F. Massel, A. Kantian, A.D. Daley, T. Giamarchi, and P. Törmä, Dynamics of an impurity in a one-dimensional lattice, submitted to New J. Phys. (2012), arXiv:1210.4270.

Speaker: Prof. Päivi Törmä (Aalto University)

Slides Torma_Nordita13.pdf

15:00 → 15:30 Coffee break

15:30 → 16:00 Dynamical arrest of ultracold lattice fermions

We theoretically investigate the thermodynamics of an interacting inhomogeneous two-component Fermi gas in an optical lattice. Motivated by a recent experiment, Science, 327, 1621 (2010), we study the effect of the interplay between thermodynamics and strong correlations on the size of the fermionic cloud. We use dynamical mean-field theory to compute the cloud size, which in the experiment shows an anomalous expansion behavior upon increasing attractive interaction. We confirm this qualitative effect but, assuming adiabaticity, we find quantitative agreement only for weak interactions. For strong interactions we observe significant non-equilibrium effects which we attribute to a dynamical arrest of the particles due to increasing correlations.

Speaker: Dr Reza Bakhtiari (Institut für Theoretische Physik Hamburg)

17:30 → 19:30 Reception

Weekly reception

Thursday, 14 February

10:00 → 10:45 Quantum Simulators of Lattice Gauge Theories: Introduction

I will present an introduction to the theory of quantum link models, a.k.a. gauge magnets, that are particularly suitable for realizations with ultracold atoms. These models provide an alternative to the standard Wilson's method formulation of lattice gauge theories (LGT). I will derive the simplest Hamiltonian for Abelian U(1) LGT and show the simplest SU(2) extension. Implementation with Rydberg atoms will be shortly discussed. [1] L. Tagliacozzo, A. Celi, A. Zamora, and M. Lewenstein, Optical Abelian Lattice Gauge Theories, Ann. Phys. (N.Y.) 330, 160-191 (2013), arXiv:1205.0496. [2] L. Tagliacozzo, A. Celi, P. Orland, and M. Lewenstein, Simulations of non-Abelian gauge theories with optical lattices, arXiv:1211.2704.

Speaker: Prof. Maciej Lewenstein (ICFO, Barcelona)

14:00 → 14:30 2D Topological Insulators: Lattice Trapping Effects and Interactions

We investigate effects of interactions and trapping in a cold-gas realization of a 2D time-reversal invariant topological insulator. In contrast to solid-state systems, the effects of trapping and the relatively small scale of cold-gas systems can significantly effect the edge states of topological systems. By choosing explicit realizations of the Hofstadter lattice with various applied trapping potentials, we show that the important properties of the topological invariants remained unaffected, despite seemingly unfavorable conditions. Furthermore, one can observe a number of other features, at least in theoretical calculations, such as splitting and merging of edge states, along with connections between edges states and bulk bands. These connections also reveal themselves in light-Bragg spectroscopy, which we have used to demonstrate the possibility for observation of edge states in these systems. To investigate interaction effects, we have taken the proposal of the system by Goldman et al. (PRL 105, 255302, 2010) which exhibits topologically insulating phases in an optical square lattice using both real-space dynamical mean-field theory (R-DMFT) and analytical techniques. This system includes a time-invariant flux term, which emulates a spin-dependent magnetic field similar to the Hofstadter-lattice, a Rashba/Dresselhaus-like spin-orbit term, which introduces non-Abelian behavior, and a staggered super-lattice potential, which introduces non-trivial topology at half-filling. We investigate with R-DMFT the robustness of the topological phases for weak interaction, as well as transitions to magnetic order at strong interaction. We demonstrate that a critical dependence exists dependent on the number of Dirac points. Furthermore, we derive and analyze the corresponding spin-Hamiltonian, and show that the competing terms of flux and Rashba-like spin-orbit couplings produce non-trivial spiral-like orders.

Speaker: Dr Daniel Cocks (Goethe University Frankfurt)

14:30 → 15:00 Employing collapse and revival oscillations for the analysis of quantum many-body states

We argue that forcing an interacting quantum many-body system to reside after a quench far from its equilibrium state, is an important tool to reveal information on the correlations in the initial ground state. We discuss two examples in detail: [1] We investigate the collapse and revival of first-order coherence in deep optical lattices when long-range interactions are turned on and find that the first few revival peaks are strongly attenuated already for moderate values of the nearest-neighbor interaction coupling. It is shown that the conventionally employed Gutzwiller wavefunction, with only on-site number dependence of the variational amplitudes, leads to incorrect predictions for the collapse and revival oscillations within the extended Bose-Hubbard model. We provide a modified variant of the Gutzwiller ansatz, reproducing the analytically calculated time dependence of first-order coherence in the limit of zero tunneling. [2] We consider the rapid quench of a one-dimensional strongly correlated supersolid to a localized density wave (checkerboard) phase, and calculate the first-order coherence signal following the quench. It is shown that unique coherence oscillations between the even and odd sublattice sites of the checkerboard are created by the quench, which are absent when the initial state is described by a Gutzwiller product state. This is a striking manifestation of the versatility of the far-from-equilbrium and nonperturbative collapse and revival phenomenon as a microscope for quantum correlations in complex many-body states. For the present example, this opens up the possibility to discriminate experimentally between mean-field and many-body origins of supersolidity.

Speaker: Prof. Uwe Fischer (Seoul National University)

15:00 → 15:30 Coffee break

15:30 → 16:00 Ferromagnetism and Nematicity of Fermions in an Optical Flux Lattice

Ultracold atoms in Raman-dressed optical lattices allow for effective momentum-dependent interactions among single-species fermions originating from short-range s-wave interactions. These dressed-state interactions combined with very flat bands encountered in the recently introduced optical flux lattices push the Stoner instability towards weaker repulsive interactions, making it accessible with current experiments. As a consequence of the coupling between spin and orbital degrees of freedom, the magnetic phase features Ising nematic order. Reference: S. K. Baur and N. R. Cooper, arXiv:1208.6540, to be published in Phys. Rev. Lett.

Speaker: Dr Stefan Baur (University of Cambridge)

Slides nordita_Baur.pdf

16:00 → 16:30 Simulating an interacting gauge theory with ultracold Bose gases

Over the last two decades ultracold atomic gases have formed the basis for a plethora of theoretical and experimental investigations of matter at the nano Kelvin temperature regime. Condensates formed from either bosonic or fermionic matter offer a large degree of experimental control, and as such it is now possible to perform quantum simulations of various physical scenarios envisaged in other condensed matter systems, such as spin-orbit coupling, quantum magnetism and even relativistic effects1,2. Here, we will discuss how one can create artificial gauge fields for an ensemble of interacting ultracold bosonic atoms using the dressed states of the light-matter interaction. Until now, all experimental gauge potentials have been static2. We will show how to induce a U(1) interacting dynamical gauge field, such that there is an effective back-action between the gauge field and the matter field. We will then go on to construct the one-dimensional solutions for such a system, which comprises unique topological states. By performing the appropriate gauge transformation, the one-dimensional many-body equation of motion can be shown to feature a current non-linearity. The resulting non-linear equation of motion can be solved exactly to yield chiral solitons3, as well as critical particle numbers required for the onset of rotation of a condensate in a ring geometry4. References [1] J. Dalibard et al., Rev. Mod. Phys. 83, 1523 (2011). [2] Y.-J. Lin et al., Nature 471 83 (2011) [3] U. Aglietti., et al, Phys. Rev. Lett. 77, 4406 (1996). [4] M. J. Edmonds et al., arXiv 1212.0445

Speaker: Dr Matthew Edmonds (Heriot-Watt University)

Slides interacting_gauge_slides_mje.pdf

Friday, 15 February

10:00 → 10:45 TBA Rudi Grimm

TBA

Speaker: Prof. Rudi Grimm (University of Innsbruck)

14:00 → 14:30 Fractional quantum Hall states of bosons subjected to artificial gauge fields

We employ the exact diagonalization method to analyze the possibility of generating strongly correlated states in two-dimensional clouds of ultracold bosonic atoms which are subjected to a geometric gauge field created by coupling two internal atomic states to a laser beam. Tuning the gauge field strength, the system undergoes stepwise transitions between different ground states, which we describe by analytical trial wave functions, amongst them the Pfaffian, the Laughlin, and a Laughlin quasiparticle many-body state. We analyze quasihole excitations over the Laughlin and generalized Laughlin states, and show that they possess effective fractional charge and obey anyonic statistics. Finally, we study the energy gap over the Laughlin state as the number of particles is increased keeping the chemical potential fixed. The gap is found to decrease as the number of particles is increased, indicating that the observability of the Laughlin state is restricted to a small number of particles. References: [1] B. Julia-Diaz, T. Grass, N. Barberan, M. Lewenstein, New Journal of Physics, 14, 055003 (2012) [2] B. Julia-Diaz, D. Dagnino, KJ Gunter, T. Grass, N. Barberan, M. Lewenstein, J. Dalibard, Phys. Rev. A 84, 053605 (2011)

Speaker: Dr Bruno Julia-Diaz (ICFO - The Institute of Photonic Sciences)

14:30 → 15:00 Fractional quantum Hall phases of two-component Bose gases

Artificial gauge fields for cold atoms are tools for producing topological quantum states. In spinless or spin-polarized systems, cold bosons are known to support the incompressible phases from the Read-Rezayi series, containing also the famous Laughlin and Moore-Read states with anyonic or even non-Abelian quasiparticle excitations. Here we show that in the case of a pseudospin-1/2 Bose gas a generalization of this series, the so-called non-Abelian spin singlet (NASS) series, describes well the ground states at different filling factors. We have also investigated the scenario where, in addition to an external magnetic field, the gauge field mimics an intrinsic spin-orbit coupling of the Rashba type. We find a variety of different phases, which can be controlled by the strength of this coupling.

Speaker: Mr Tobias Grass (ICFO - The Institute of Photonic Science)

15:00 → 16:00 Coffee break