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arXiv2013

Beyond the Dicke Quantum Phase Transition with a Bose-Einstein Condensate in an Optical Ring Resonator

We experimentally investigate the dynamical instability of a Bose Einstein condensate in an optical ring resonator for various cavity detuning and pump powers. The resulting phase diagram is asymmetric with respect to the cavity detuning and can be described by the coupling of two atomic modes with one optical mode. We compare the experimental data to a numerical simulation and to an analytic expression of the phase boundary. For positive and negative pump cavity detuning different coupling mechanisms are identified explaining the asymmetry of the phase diagram. We present a physical interpretation and discuss the connection to the Dicke quantum phase transition.

physics.atom-ph↗

arXiv2013

Dynamics of BEC mixtures loaded into the optical lattice in the presence of linear inter-component coupling

We consider dynamics of a two-component Bose-Einstein condensate where the components correspond to different hyperfine states of the same sort of atoms. External microwave radiation leads to resonant transitions between the states. The condensate is loaded into the optical lattice. We invoke the tight-binding approximation and examine the interplay of spatial and internal dynamics of the mixture. It is shown that internal dynamics qualitatively depends on the intra-component interaction strength and the phase configuration of the initial state. We focus attention on two intriguing phenomena occurring for certain parameter values. The first phenomenon is the spontaneous synchronization of Rabi oscillations running inside neighbouring lattice sites. Another one is the demixing of the condensate with formation of immiscible solitons when nonlinearity becomes sufficiently strong. Demixing is preceded by the transient regime with highly irregular behavior of the mixture.

cond-mat.quant-gas↗

arXiv2013

Excitations of optomechanically driven Bose-Einstein condensates in a cavity: photodetection measurements

We present a detailed study to analyse the Dicke quantum phase transition within the thermodynamic limit for an optomechanically driven Bose-Einstein condensates in a cavity. The photodetection-based quantum optical measurements have been performed to study the dynamics and excitations of this optomechanical Dicke system. For this, we discuss the eigenvalue analysis, fluorescence spectrum and the homodyne spectrum of the system. It has been shown that the normal phase is negligibly affected by the mechanical mode of the mirror while it has a significant effect in the superradiant phase. We have observed that the eigenvalues and both the spectra exhibit distinct features that can be identified with the photonic, atomic and phononic branches. In the fluorescence spectra, we further observe an asymmetric coherent energy exchange between the three degrees of freedom of the system in the superradiant phase arising as a result of optomechanical interaction and Bloch-Siegert shift.

cond-mat.quant-gas↗

arXiv2013

Topological insulator states in quasi-one-dimensional sawtooth chain

We report the discovery of flatten Bloch bands with nontrivial topological numbers in a quasi-one-dimensional sawtooth chain. We present the nearly flat-band with a obvious gap and nonzero Chern number of the modulated sawtooth chain by tuning hoppings and modulation. With the increasing of the strength of the modulation, the system undergos a series of topological phase transitions in the absence of interaction. By adding interaction to the model, we present exact diagonalization results for the system at $1/3$ filling and some interesting connections with fraction quantum Hall states: topological degeneracy, nontrivially topological number and fractional statistic of the quasihole.

cond-mat.str-el↗

arXiv2013

Hydrodynamic theory of motion of quantized vortex rings in trapped superfluid gases

I study vortex ring oscillations in a superfluid, trapped in an elongated trap, under the conditions of the Local Density Approximation. On the basis of the Hamiltonian formalism I develop a hydrodynamic theory, which is valid for an arbitrary superfluid and depends only on the equation of state. The problem is reduced to an ordinary differential equation for the ring radius. The cases of the dilute BEC and the Fermi gas at unitarity are investigated in detail. Simple analytical equations for the periods of small oscillations are obtained and the equations of non-linear dynamics are solved in quadratures. The results agree with available numerical calculations. Experimental possibilities to check the predictions are discussed.

cond-mat.quant-gas↗

arXiv2013

Quantum Simulation of Non-Abelian Lattice Gauge Theories

We use quantum link models to construct a quantum simulator for U(N) and SU(N) lattice gauge theories. These models replace Wilson's classical link variables by quantum link operators, reducing the link Hilbert space to a finite number of dimensions. We show how to embody these quantum link models with fermionic matter with ultracold alkaline-earth atoms using optical lattices. Unlike classical simulations, a quantum simulator does not suffer from sign problems and can thus address the corresponding dynamics in real time. Using exact diagonalization results we show that these systems share qualitative features with QCD, including chiral symmetry breaking and we study the expansion of a chirally restored region in space in real time.

hep-lat↗

arXiv2013

Vortex Rings in a Trap

We present a simple Hamiltonian description of the dynamics of a quantized vortex ring in a trapped superfluid, compare this description with dynamical simulations, and characterize the dependence of the dynamics of the shape of the trap.

cond-mat.quant-gas↗

arXiv2013

Inverse population transfer of the repulsive Bose-Einstein condensate in a double-well trap: strong interaction-induced support

An inverse population transfer of the repulsive Bose-Einstein condensate (BEC) in a weakly bound double-well trap is explored within the 3D time-dependent Gross-Pitaevskii equation. The model avoids numerous common approximations (two-mode treatment, time-space factorization, etc) and closely follows the conditions of Heidelberg experiments, thus providing a realistic description of BEC dynamics. The transfer is driven by a time-dependent shift of a barrier separating the left and right wells. It is shown that completeness and robustness of the process considerably depend on the amplitude and time profile of the shift velocity. Soft profiles provide the most robust inversion. The repulsive interaction substantially supports the transfer making it possible i) in a wide velocity interval and ii) three orders of magnitude faster than in the ideal BEC.

cond-mat.quant-gas↗

arXiv2013

Impact of local integrals of motion to metastable non-equilibrium states

We analyse the stationary behaviour of correlations in a strongly correlated Bose gas out of equilibrium. The dynamics are triggered by a quench of the interaction starting from the strongly interacting limit where the system is in a perfect Mott state. Despite the complete integrability of our theoretical description, we find seemingly thermal behaviour for the experimentally measurable correlations at large interactions. Quite opposed, away from the strongly interacting regime these correlation functions show highly non-thermal stationary values. This behaviour is explained by overlaps of the integrals of motion with the observable and the initial state in an effective thermal ensemble. These results suggest that non-equilibrium Mazur equalities are an efficient way to calculate short range correlations for arbitrary integrable models.

cond-mat.quant-gas↗

arXiv2013

Quantum gases in optical lattices

The experimental realization of correlated quantum phases with ultracold gases in optical lattices and their theoretical understanding has witnessed remarkable progress during the last decade. In this review we introduce basic concepts and tools to describe the many-body physics of quantum gases in optical lattices. This includes the derivation of effective lattice Hamiltonians from first principles and an overview of the emerging quantum phases. Additionally, state-of-the-art numerical tools to quantitatively treat bosons or fermions on different lattices are introduced.

cond-mat.quant-gas↗

arXiv2013

Reply to Comment on "Quantum quasicrystals of spin-orbit coupled dipolar bosons''

In a recent Letter [Phys. Rev. Lett. 111, 185304 (2013)], we proposed a scheme for realizing quantum quasicrystals using spin-orbit coupled dipolar bosons. We remarked that these quantum quasicrystals have additional ``phason''-like modes compared with their classical counterparts. A recent comment by Lifshitz [arXiv:1312.1388] contests this claim. We argue here that our enumeration of gapless modes is indeed the physically relevant one; whether the additional modes are ``phasons'' is, however, a matter of definition.

cond-mat.quant-gas↗

arXiv2013

Non-Fermi liquid behavior in Bose-Fermi mixtures at two dimensions

In this paper we study the low temperature behaviors of a system of Bose-Fermi mixtures at two dimensions. Within a self-consistent ladder diagram approximation, we show that at nonzero temperatures $T\rightarrow0$ the fermions exhibit non-fermi liquid behavior. We propose that this is a general feature of Bose-Fermi mixtures at two dimensions. An experimental signature of this new state is proposed.

cond-mat.str-el↗

arXiv2013

Tunable lattice Induced Opacity in atom-lattice interaction

A lattice-induced opacity is identified in the scattering process of a normally-incident matter wave from a two dimensional lattice of atoms. This system can be treated as an analogue of a confinement induced resonance. Specifically by modifying the s-wave scattering length between atoms in the incident matter wave and the lattice-confined atoms, the transmission of the matter wave can be tuned controllably from zero to one. Based on this, we propose a possible scheme for a matter wave transistor. When the transmission of the matter wave is tuned to zero, a matter wave cavity can be formed by placing two such lattice planes of atoms parallel to one another. At higher kinetic energies, the two dimensional lattice of atoms can also serve as a matter wave beam splitter and a wave plate.

physics.atom-ph↗

arXiv2013

Artificial Gauge Fields and Spin-Orbit Couplings in Cold Atom Systems

This article is a report of Projet bibliographique of M1 at École Normale Supérieure. In this article we reviewed the historical developments in artificial gauge fields and spin-orbit couplings in cold atom systems. We resorted to origins of literatures to trace the ideas of the developments. For pedagogical purposes, we tried to work out examples carefully and clearly, to verified the validity of various approximations and arguments in detail, and to give clear physical and mathematical pictures of the problems that we discussed. The first part of this article introduced the fundamental concepts of Berry phase and Jaynes-Cummings model. The second part reviewed two schemes to generate artificial gauge fields with N-pod scheme in cold atom systems. The first one is based on dressed-atom picture which provide a method to generate non-Abelian gauge fields with dark states. The second one is about rotating scheme which is achieved earlier historically. Non-Abelian gauge field inevitably leads to spin-orbit coupling. We reviewed some developments in achieve spin-orbital coupling theoretically and experimentally. The fourth part was devoted to recently developed idea of optical flux lattice that provides a possibility to reach the strongly correlated regime in cold atom systems. We developed a geometrical interpretation based on Cooper's theory. Some useful formulae and their proofs were listed in the Appendix.

cond-mat.quant-gas↗

arXiv2014

A tight binding model for quantum spin Hall effect on triangular optical lattice

We propose a tight binding model for the quantum spin Hall system on triangular optical lattice and we determined the edge state spectrum which contains gap traversing states as the hallmark of $\mathds{Z}_{2}$ topological insulator. The advantage of this system is the possibility of implementing it in the fermionic ultracold atomic system whose nearly free electron limit is proposed by B. Béri and N. R. Cooper, Phys. Rev. Lett. {\bf 107}, 145301 (2011).

cond-mat.mes-hall↗

arXiv2014

Hybrid quantum systems of ions and atoms

In this chapter we review the progress in experiments with hybrid systems of trapped ions and ultracold neutral atoms. We give a theoretical overview over the atom-ion interactions in the cold regime and give a summary of the most important experimental results. We conclude with an overview of remaining open challenges and possible applications in hybrid quantum systems of ions and neutral atoms.

cond-mat.quant-gas↗

arXiv2014

Quasi-Particle Theory for the Higgs Amplitude Mode

We present a generalized quasi-particle theory for bosonic lattice systems, which naturally contains all relevant collective modes, including the Higgs amplitude in the strongly correlated superfluid. In contrast to Bogoliubov theory, this non-perturbative method does not rely on a small condensate depletion and is valid for any interaction strength in three spatial dimensions. It is based on an expansion around the bosonic Gutzwiller ground state in terms of appropriately chosen fluctuation operators and lays the foundation for the description of real-time dynamics in terms of the natural, weakly interacting quasi-particles. Furthermore, it provides a systematic framework for efficiently calculating observables beyond the Gutzwiller approximation and for including external perturbations, as well as higher order decay and interactions in terms of quasi-particle operators. It allows for the construction of an alternative path integral approach in terms of quasi-particle coherent states.

cond-mat.quant-gas↗

arXiv2014

Influence of finite size of molecules on spectrum of fully polarized dipolar BECs: Generalization of Bogoliubov's spectrum

We discuss complete theory of point-like particles in fully polarized BECs describing difference in behaviour of electrically- and magnetically polarized BECs. Next we present generalization of this theory on finite size particles to include contribution of size of real molecules on dynamical properties of dipolar BECs. As an application of obtained equation we calculate spectrum of linear collective excitations getting generalization of the Bogoliubov's spectrum. We show absence of roton instability due to positivity of polarization contribution in the excitation spectrum. We found that new type of instabilities appears in small wave length limit due to finite size of molecules.

cond-mat.quant-gas↗