Dec 24 - Dec 28, Xiaopeng Li
Dec 27
1. arXiv:1212.6224 [pdf, other]
Multi-Terminal Coulomb-Majorana Junction
Alexander Altland, Reinhold Egger
We study multiple helical nanowires in proximity to a common mesoscopic superconducting island, where Majorana fermion bound states are formed. We show that a weak finite charging energy of the center island may dramatically affect the low-energy behavior of the system. While for strong charging interactions, the junction decouples the connecting wires, interactions lower than a non-universal threshold may trigger the flow towards an exotic Kondo fixed point. In either case, the ideally Andreev reflecting fixed point characteristic for infinite capacitance (grounded) devices gets destabilized by interactions.
2. arXiv:1212.6191 [pdf, other]
Statistical Topological Insulators
I. C. Fulga, B. van Heck, J. M. Edge, A. R. Akhmerov
We define a class of insulators with gapless surface states protected from localization due to the statistical properties of a disordered ensemble, namely due to the ensemble's invariance under a certain symmetry. We show that these insulators are topological, and are protected by a Z2 invariant. Finally, we prove that every topological insulator gives rise to an infinite number of classes of statistical topological insulators in higher dimensions. Our conclusions are confirmed by numerical simulations.
3.arXiv:1212.5947 [pdf, ps, other]Controlling the density of the 2DEG at the SrTiO3/LaAlO3 interface
A. Janotti, L. Bjaalie, L. Gordon, C. G. Van de Walle
The polar discontinuity at the SrTiO3/LaAlO3 interface (STO/LAO) can in principle sustain an electron density of 3.3E14 cm-2 (0.5 electrons per unit cell). However, experimentally observed densities are more than an order of magnitude lower. Using a combination of first-principles and Schrodinger-Poisson simulations we show that the problem lies in the asymmetric nature of the structure, i.e., the inability to form a second LAO/STO interface that is a mirror image of the first, or to fully passivate the LAO surface. Our insights apply to oxide interfaces in general, explaining for instance why the SrTiO3/GdTiO3 interface has been found to exhibit the full density of 3.3E14 cm-2.
4. arXiv:1212.5670 [pdf, other]
Majorana fermion description of the Kondo lattice: Variational and Path integral approach
Johan Nilsson, Matteo Bazzanella
All models of interacting electrons and spins can be reformulated as theories of interacting Majorana fermions. We consider the Kondo lattice model that admits a symmetric representation in terms of Majorana fermions. In the first part of this work we study two variational states, which are natural in the Majorana formulation. At weak coupling a state in which three Majorana fermions tend to propagate together as bound objects is favored, while for strong coupling a better description is obtained by having deconfined Majorana fermions. This way of looking at the Kondo lattice offers an alternative phenomenological description of this model. In the second part of the paper we provide a detailed derivation of the discretized path integral formulation of any Majorana fermion theory. This general formulation will be useful as a starting point for further studies, such as Quantum Monte Carlo, perturbative expansions, and Renormalization Group analysis. As an example we use this path integral formalism to formulate a finite temperature variational calculation, which generalizes the ground state variational calculation of the first part. This calculation shows how the formation of three-body bound states of Majorana fermions can be handled in the path integral formalism.
5. arXiv:1212.5598 [pdf, other]
Topological protection of bound states against the hybridization
Bohm-Jung Yang, Mohammad Saeed Bahramy, Naoto Nagaosa
Topological invariants are conventionally known to be responsible for protection of extended states against disorder. A prominent example is the presence of topologically protected extended-states in two-dimensional (2D) quantum Hall systems as well as on the surface of three-dimensional (3D) topological insulators. Distinct from such cases, here we introduce a new concept, that is, the topological protection of bound states against hybridization. This situation is shown to be realizable in a 2D quantum Hall insulator put on a 3D trivial insulator. In such a configuration, there exist topologically protected bound states, localized along the normal direction of 2D plane, in spite of hybridization with the continuum of extended states. The one-dimensional edge states are also localized along the same direction as long as their energies are within the band gap. This finding demonstrates the dual role of topological invariants, as they can also protect bound states against hybridization in a continuum.
6. arXiv:1212.3666 (cross-list from hep-th) [pdf, ps, other]
Spontaneous Generation of Angular Momentum in Holographic Theories
Hong Liu, Hirosi Ooguri, Bogdan Stoica, Nicolas Yunes
The Schwarzschild black two-brane in AdS_4 is dual to a finite temperature state in CFT_3. We show that the solution acquires a non-zero angular momentum density when a gravitational Chern-Simons coupling is turned on in the bulk, even though the solution is not modified. A similar phenomenon is found for the Reissner-Nordstrom black two-brane with axionic coupling to the gauge field. We discuss interpretation of this phenomenon from the point of view of the boundary CFT_3.
Dec 24
1. arXiv:1212.5379 [pdf, ps, other]
Closed system approach to open systems: Tunneling decay of interacting cold bosons in an optical lattice
K. Rapedius
A Bose-Hubbard Hamiltonian, modeling cold bosons in an optical lattice, is used to simulate the dynamics of interacting open quantum systems as subsystems a larger closed system, avoiding complications like the introduction of baths, complex absorbing potentials or absorbing boundaries. The numerically exact unitary dynamics is compared with effective descriptions of the subsystems based on non-Hermitian Hamiltonians or Lindblad master equations. The validity of popular models with constant decay rates is explicitly analyzed for decaying single and double wells. In addition we present a discrete lattice version of the Siegert approximation method for calculating decay rates.
2. arXiv:1212.5469 [pdf, ps, other]
Resonant demagnetization of a dipolar BEC in a 3D optical lattice
A. de Paz, A. Chotia, E. Marechal, P. Pedri, L. Vernac, O. Gorceix, B. Laburthe-Tolra
We study dipolar relaxation of a chromium BEC loaded into a 3D optical lattice. We observe dipolar relaxation resonances when the magnetic energy released during the inelastic collision matches an excitation towards higher energy bands. A spectroscopy of these resonances for two orientations of the magnetic field provides a 3D band spectroscopy of the lattice. The narrowest resonance is registered for the lowest excitation energy. Its line-shape is sensitive to the on-site interaction energy. We use such sensitivity to probe number squeezing in a Mott insulator, and we reveal the production of three-body states with entangled spin and orbital degrees of freedom.
Dec 17 - Dec 21, Saubhik Sarkar
Dec 21
1. arXiv:1212.5093 [pdf, other]
Direct imaging of topological edges states with cold atoms
N. Goldman, J. Dalibard, A. Dauphin, F. Gerbier, M. Lewenstein, P. Zoller, I. B. Spielman
Detecting topological order in cold-atom experiments is an outstanding challenge, the resolution of which offers novel perspectives on topological matter. In material systems, unambiguous signatures of topological order exist for topological insulators and quantum Hall devices. In quantum Hall systems, the quantized conductivity and the associated robust propagating edge modes - guaranteed by the existence of non-trivial topological invariants - have been observed through transport and spectroscopy measurements. Here, we show that optical-lattice-based experiments can be tailored to directly visualize the propagation of topological edge modes. Our method is rooted in the unique capability for initially shaping the atomic gas, and imaging its time-evolution after suddenly removing the shaping potentials. Our scheme, applicable to an assortment of atomic topological phases, provides the first method for imaging the dynamics of topological edge modes, directly revealing their angular velocity and spin structure.
2. arXiv:1212.4839 (cross-list from cond-mat.str-el) [pdf, other]
Realizing Fractional Chern Insulators with Dipolar Spins
Norman Y. Yao, Alexey V. Gorshkov, Chris R. Laumann, Andreas M. Läuchli, Jun Ye, Mikhail D. Lukin
Strongly correlated quantum systems can exhibit exotic behavior controlled by topology. We predict that the \nu=1/2 fractional Chern insulator arises naturally in a two-dimensional array of driven, dipolar-interacting spins. As a specific implementation, we analyze how to prepare and detect synthetic gauge potentials for the rotational excitations of ultra-cold polar molecules trapped in a deep optical lattice. While the orbital motion of the molecules is pinned, at finite densities, the rotational excitations form a fractional Chern insulator. We present a detailed experimental blueprint for KRb, and demonstrate that the energetics are consistent with near-term capabilities. Prospects for the realization of such phases in solid-state dipolar systems are discussed as are their possible applications.
Dec 20
1. arXiv:1212.4824 [pdf, ps, other]
Controlling the dynamics of an open many-body quantum system with localized dissipation
G. Barontini, R. Labouvie, F. Stubenrauch, A. Vogler, V. Guarrera, H. Ott
We experimentally investigate the action of a localized dissipative potential on a macroscopic matter wave, which we implement by shining an electron beam on an atomic Bose-Einstein condensate (BEC). We measure the losses induced by the dissipative potential as a function of the dissipation strength observing a paradoxical behavior when the strength of the dissipation exceeds a critical limit: for an increase of the dissipation rate the number of atoms lost from the BEC becomes lower. We repeat the experiment for different parameters of the electron beam and we compare our results with a simple theoretical model, finding excellent agreement. By monitoring the dynamics induced by the dissipative defect we identify the mechanisms which are responsible for the observed paradoxical behavior. We finally demonstrate the link between our dissipative dynamics and the measurement of the density distribution of the BEC allowing for a generalized definition of the Zeno effect. Due to the high degree of control on every parameter, our system is a promising candidate for the engineering of fully governable open quantum systems.
2.arXiv:1212.4637 [pdf, ps, other]
Emergence of glass-like dynamics for dissipative and strongly interacting bosons
Dario Poletti, Peter Barmettler, Antoine Georges, Corinna Kollath
We study the dynamics of a strongly interacting bosonic quantum gas in an optical lattice potential under the effect of a dissipative environment. We show that the interplay between the dissipative process and the Hamiltonian evolution leads to an unconventional dynamical behavior of observables, such as local number fluctuations. In particular we show, both analytically and numerically, the emergence of an anomalous diffusive evolution at short times and, at long times, a glass-like dynamics dominated by rare events. This complex dynamics reveals information on the level structure of the strongly interacting gas.
3. arXiv:1212.4570 [pdf, ps, other]
Mott-insulator phases at half-integer fillings in the imbalanced honeycomb lattice
Krzysztof Gawryluk, Christian Miniatura, Benoît Grémaud
We investigate the ground state properties, using an improved Gutzwiller-like ansatz, of ultracold bosons loaded in a honeycomb lattice, with on-site repulsive interaction. In the situation of imbalanced hopping amplitudes, we have found half-integer Mott-insulator phases, i.e. states with half-integer filling and vanishing compressibility, arising due to the interplay between the quantum correlations and the topology of the honeycomb lattice. These new phases have unambiguous signatures in the velocity distribution, and thus could be easily observed experimentally.
Dec 19
1. arXiv:1212.4478 [pdf, ps, other]
Complex phases in the doped two-species bosonic Hubbard Model
Kalani Hettiarachchilage, Valéry G. Rousseau, Ka-Ming Tam, Mark Jarrell, Juana Moreno
We study a two-dimensional bosonic Hubbard model with two hard-core species away from half filling using Quantum Monte Carlo simulations. The model includes a repulsive interspecies interaction and different nearest-neighbor hopping terms for the two species. By varying the filling we find a total of five distinct phases, including a normal liquid phase at higher temperature, and four different phases at lower temperature. We find an anti-ferromagnetically ordered Mott insulator and a region of coexistent anti-ferromagnetic and superfluid phases near half filling. Further away from half filling the phase diagram displays a superfluid phase and a novel phase inside the superfluid region at even lower temperatures. In this novel phase separated region, the heavy species has a Mott behavior with integer filling, while the lighter species shows phase separated Mott and superfluid behaviors.
2. arXiv:1212.4453 [pdf, other]
Bose-Einstein condensation of atoms in a uniform potential
Alexander L. Gaunt, Tobias F. Schmidutz, Igor Gotlibovych, Robert P. Smith, Zoran Hadzibabic
We have observed Bose-Einstein condensation of an atomic gas in the (quasi-)uniform three-dimensional potential of an optical box trap. Condensation is seen in the bimodal momentum distribution and the anisotropic time-of-flight expansion of the condensate. The critical temperature agrees with the theoretical prediction for a uniform Bose gas. The momentum distribution of our non-condensed quantum-degenerate gas is also clearly distinct from the conventional case of a harmonically trapped sample and close to the expected distribution in a uniform system. We confirm the coherence of our condensate in a matter-wave interference experiment. Our experiments open many new possibilities for fundamental studies of many-body physics.
3.arXiv:1212.4263 [pdf, ps, other]
Coherence and dynamics of a two-component condensate coupling with a continuum
Honghua Zhong, Qiongtao Xie, Jun Xu, Wenhua Hai, Chaohong Lee
We investigate the many-body coherence and population dynamics of a two-component atomic Bose-Einstein condensate coupled with a continuum. We reveal that the combination of dissipation and nonlinearity may induce different steady states and modify the bifurcation of coherence. The steady states depend on the relative dissipation strengths. The coherence may be even enhanced by the dissipation effects. The combination of dissipation and nonlinearity allows one to control the switching between different self-trapped states or the switching between a self-trapped state and a non-self-trapped state. This opens an alternative route for manipulating coherence and population transition in an open many-body system via dissipation and nonlinearity.
4. arXiv:1212.4254 [pdf, ps, other]
Dissipative quantum systems: from two to many atoms
Dario Poletti, Jean-Sebastien Bernier, Antoine Georges, Corinna Kollath
We study the dynamics of bosonic atoms in a double well potential under the influence of dissipation. The main effect of dissipation is to destroy quantum coherence and to drive the system towards a unique steady state. We study how the atom-atom interaction affects the decoherence process. We use a systematic approach considering different atomic densities. We show that, for two atoms, the interaction already strongly suppresses decoherence: a phenomenon we refer to as "interaction impeded decoherence". For many atoms, thanks to the increased complexity of the system, the nature of the decoherence process is dramatically altered giving rise to an algebraic instead of exponential decay.
5.arXiv:1212.4485 (cross-list from cond-mat.str-el) [pdf, ps, other]
Mott transitions in the half-filled SU(2M) symmetric Hubbard model
N. Blümer, E. Gorelik
The Hubbard model with large orbital degeneracy has recently gained relevance in the context of ultracold earth alkali like atoms. We compute its static properties in the SU(2M) symmetric limit for up to M=8 bands at half filling within dynamical mean-field theory, using the numerically exact multigrid Hirsch-Fye quantum Monte Carlo approach. Based on this unbiased data, we establish scaling laws which predict the phase boundaries of the paramagnetic Mott metal-insulator transition at arbitrary orbital degeneracy M with high accuracy.
Dec 18
1. arXiv:1212.3762 [pdf, ps, other]
Excitation Spectrum and Momentum Distribution of Bose-Hubbard Model with On-site Two- and Three-body Interaction
Beibing Huang, Shaolong Wan
An effective action for Bose-Hubbard model with two- and three-body on-site interaction in a square optical lattice is derived in the frame of a strong-coupling approach developed by Sengupta and Dupuis. From this effective action, superfluid-Mott insulator (MI) phase transition, excitation spectrum and momentum distribution for two phases are calculated by taking into account Gaussian fluctuation about the saddle-point approximation. In addition the effects of three-body interaction are also discussed.
Dec 17
1.arXiv:1212.3486 [pdf, other]
Time-evolution of excitations in normal Fermi liquids
Y. Pavlyukh, A. Rubio, J. BerakdarWe inspect the initial and the long time evolution of excitations a Fermi liquids by analyzing the time behavior of the electron spectral function. Focusing on the short-time limit we study the electron-boson model for the homogenous electron gas and apply the first order (in boson propagator) cumulant expansion of the electron Green's function. In addition to a quadratic decay in time upon triggering the excitation, we identify non-analytic terms in the time expansion similar to those found in the Fermi edge singularity phenomenon. We also demonstrate that the exponential decay in time in the long-time limit is inconsistent with the GW approximation for the self-energy. The background for this is the Paley-Wiener theorem of complex analysis. To reconcile with the Fermi liquid behavior an inclusion of higher order diagrams (in the screened Coulomb interaction) is required.
2.arXiv:1212.3535 [pdf, ps, other]
Dynamical response of ultracold interacting fermion-boson mixtures: Fermion-hole vs polaron-hole excitations
Stefan Maier, Kai Ji, Andreas KomnikWe analyze the dynamical response of a ultracold binary gas mixture in presence of strong boson-fermion couplings. Mapping the problem onto that of the optical response of a metal/semiconductor electronic degrees of freedom to electromagnetic perturbation we calculate the corresponding linear response susceptibility in the non-perturbative regimes of strong boson-fermion coupling using diagrammatic resummation technique as well as quantum Monte Carlo simulations. Depending on the interaction strength and the details of the underlying bosonic spectra we find that energy absorbtion/emission can occur due to excitation of individual fermions as well as fully dressed polarons.
3.arXiv:1212.3565 [pdf, ps, other]
Single-branch theory of ultracold Fermi gases with artificial Rashba spin-orbit coupling
Daniel Maldonado-Mundo, Patrik Ohberg, Manuel ValienteWe consider interacting ultracold fermions subject to Rashba spin-orbit coupling. We construct a single-branch interacting theory for the Fermi gas when the system is dilute enough so that the positive helicity branch is not occupied at all in the non-interacting ground state. We show that the theory is renormalizable in perturbation theory and therefore yields a model of polarized fermions that avoids a multi-channel treatment of the problem. Our results open the path towards a much more straightforward approach to the many-body physics of cold atoms subject to artificial vector potentials.
4.arXiv:1212.3594 [pdf, other]
Bloch Oscillations of Cold Atoms in a Cavity: Effects of Quantum Noise
B. Prasanna Venkatesh, D. H. J. O'Dell
In this communication we extend our theory of Bloch oscillations of cold atoms inside an optical cavity [Venkatesh et al., Phys. Rev. A 80, 063834 (2009)] to include the effects of quantum noise. By solving the coupled dynamics of linearized fluctuations about the atomic and optical meanfields, we are able to include the effects of quantum measurement backaction upon the atoms and ultimately examine how this influences the signal-to-noise ratio of a measurement of external forces using this system. One of the hurdles we overcome along the way is the proper treatment of fluctuations about time-dependent meanfields in the cold atom cavity-QED context.
Dec 10 - Dec 14, Johannes Schachenmayer
Fri Dec 14th:
1. arXiv:1212.2983 [pdf, other]
Tensor network study of the Shastry-Sutherland model in zero magnetic field
Philippe Corboz, Frederic Mila
Comments: 11 pages, 11 figuresSubjects: Strongly Correlated Electrons (cond-mat.str-el)
We simulate the Shastry-Sutherland model in two dimensions by means of infinite projected entangled-pair states (iPEPS) - a variational tensor network method where the accuracy can be systematically controlled by the so-called bond dimension. Besides the well established dimer and N\'eel phase iPEPS confirms the presence of an intermediate phase with plaquette long-range order, and we determine its phase boundaries with high accuracy. The first order phase transition for $J=0.675(2)$ between dimer and plaquette phase is compatible with previous series expansion results. iPEPS predicts a weak first-order phase transition between plaquette and N\'eel phase occurring for $J=0.765(15)$. We do not find a stable intermediate columnar-dimer phase, even when we bias the state towards this order.
2. arXiv:1212.3059 [pdf, ps, other]
Realizing an $n$-target-qubit controlled phase gate in cavity QED: An approach without classical pulses
Qi-Ping Su, Man Liu, Chui-Ping Yang
Comments: 9 pages, 5 figures, accepted by Progress of Theoretical and Experimental PhysicsSubjects: Quantum Physics (quant-ph); Superconductivity (cond-mat.supr-con)
We propose a way to realize a multiqubit controlled phase gate with one qubit simultaneously controlling $n$ target qubits using atoms in cavity QED. In this proposal, there is no need of using classical pulses during the entire gate operation. The gate operation time scales as $\sqrt{n}$ only and thus the gate can be performed faster when compared with sending atoms through the cavity one at a time. In addition, only three steps of operations are required for realizing this $n$-target-qubit controlled phase gate. This proposal is quite general, which can be applied to other physical systems such as various superconducting qubits coupled to a resonator, NV centers coupled to a microsphere cavity or quantum dots in cavity QED.
Thu Dec 13th:
1. arXiv:1212.2634 [pdf, other]
Quantum magnetism of ultracold fermions in an optical lattice
Daniel Greif, Thomas Uehlinger, Gregor Jotzu, Leticia Tarruell, Tilman Esslinger
Comments: 10 pages, 9 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
Quantum magnetism lies at the heart of many intriguing phenomena in condensed matter physics. Its manifestations range from antiferromagnets to spin-liquids, and it is believed to play a central role in high-temperature superconductivity [1-3]. Remarkably, even simple models of the underlying many-body physics are often intractable with current theoretical and computational methods. The controlled setting of ultracold fermionic atoms in optical lattices is therefore regarded as a promising route to provide new insights [4-6]. Yet, the low temperature scale required for entering the regime of quantum magnetism has hindered progress for optical lattice based systems. So far, superexchange oscillations on isolated double wells, one-dimensional decoupled Ising spin chains and classical magnetism on a triangular lattice were studied with bosonic quantum gases [7-9]. Here we report on the observation of quantum magnetism of a Fermi gas in an optical lattice. The key to obtaining and detecting the short-range magnetic order is a tunable geometry optical lattice. 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.
2. arXiv:1212.2907 [pdf, other]
Landau-Zener Transitions in Chains
N. A. Sinitsyn
Comments: 8 pages, 7 figuresSubjects: Quantum Gases (cond-mat.quant-gas); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Mathematical Physics (math-ph)
We determine transition probabilities in two exactly solvable multistate Landau-Zener (LZ) models and discuss applications of our results to the theory of dynamic passage through a phase transition in the dissipationless quantum mechanical regime. In particular, we show that statistics of particles in a new phase demonstrate scaling behavior. Our results also reveal a symmetry that we claim is a property of a large class of multistate LZ models, whose explicit solutions are not presently known. We support our arguments by direct numerical simulations.
3. arXiv:1212.2911 [pdf, other]
Experimental realization of strong effective magnetic fields in optical superlattice potentials
Monika Aidelsburger, Marcos Atala, Sylvain Nascimbène, Stefan Trotzky, Yu-Ao Chen, Immanuel Bloch
Comments: Extended version of Phys. Rev. Lett. 107, 255301 (2011) [arXiv:1110.5314]Subjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
We present the experimental generation of large effective magnetic fields for ultracold atoms using photon-assisted tunneling in an optical superlattice. The underlying method does not rely on the internal structure of the atoms and therefore constitutes a general approach to realize widely tunable artificial gauge fields without the drawbacks of near-resonant optical potentials. When hopping in the lattice, the accumulated phase shift by an atom is equivalent to the Aharonov-Bohm phase of a charged particle exposed to a staggered magnetic field of large magnitude, on the order of one flux quantum per plaquette. We study the ground state of this system and observe that the frustration induced by the magnetic field can lead to a degenerate ground state for non-interacting particles. We provide a local measurement of the phase acquired by single particles due to photon-assisted tunneling. Furthermore, the quantum cyclotron orbit of single atoms in the lattice exposed to the effective magnetic field is directly revealed.
Wed Dec 12th:
1. arXiv:1212.2219 [pdf, other]
Scaling of the gap, fidelity susceptibility, and Bloch oscillations across the superfluid to Mott insulator transition in the one-dimensional Bose-Hubbard model
Juan Carrasquilla, Salvatore R. Manmana, Marcos Rigol
Comments: 5 pages.Submitted to PRASubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas)
We investigate the interaction induced superfluid to Mott insulator transition in the one-dimensional (1D) Bose-Hubbard model (BHM) for fillings $n=1$, $n=2$, and $n=3$ by studying the single-particle gap, the fidelity susceptibility, and the amplitude of Bloch oscillations via density matrix renormalization group (DMRG) methods. We apply a generic scaling procedure for the gap, which allows us to determine the critical points with very high accuracy. We also study how the fidelity susceptibility behaves across the phase transition. Furthermore, we show that in the BHM, and in a system of spinless fermions, the amplitude of Bloch oscillations after a tilt of the lattice vanishes at the critical points. This indicates that Bloch oscillations can serve as a tool to detect the transition point in ongoing experiments with ultracold gases.
2. arXiv:1212.2539 [pdf, other]
Production of quantum degenerate strontium gases: Larger, better, faster, colder
Simon Stellmer, Rudolf Grimm, Florian Schreck
Comments: 18 pages, 10 figures, title music by Daft PunkSubjects: Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
We report on an improved scheme to generate Bose-Einstein condensates (BECs) and degenerate Fermi gases of strontium. This scheme allows us to create quantum gases with higher atom number, a shorter time of the experimental cycle, or deeper quantum degeneracy than before. We create a BEC of 84-Sr exceeding 10^7 atoms, which is a 30-fold improvement over previously reported experiments. We increase the atom number of 86-Sr BECs to 2.5x10^4 (a fivefold improvement), and refine the generation of attractively interacting 88-Sr BECs. We present a scheme to generate 84-Sr BECs with a cycle time of 2s, which, to the best of our knowledge, is the shortest cycle time of BEC experiments ever reported. We create deeply-degenerate 87-Sr Fermi gases with T/T_F as low as 0.10(1), where the number of populated nuclear spin states can be set to any value between one and ten. Furthermore, we report on a total of five different double-degenerate Bose-Bose and Bose-Fermi mixtures. These studies prepare an excellent starting point for applications of strontium quantum gases anticipated in the near future.
3. arXiv:1212.2418 [pdf, other]
Quantum simulation of open-system dynamical maps with trapped ions
P. Schindler, M. Müller, D. Nigg, J. T. Barreiro, E. A. Martinez, M. Hennrich, T. Monz, S. Diehl, P. Zoller, R. Blatt
Comments: 8 pages of main text containing 5 color figures, complemented by extensive and self-contained appendicesSubjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas)
Dynamical maps describe general transformations of the state of a physical system, and their iteration can be interpreted as generating a discrete time evolution. Prime examples include classical nonlinear systems undergoing transitions to chaos. Quantum mechanical counterparts show intriguing phenomena such as dynamical localization on the single particle level. Here we extend the concept of dynamical maps to an open-system, many-particle context: We experimentally explore the stroboscopic dynamics of a complex many-body spin model by means of a universal quantum simulator using up to five ions. In particular, we generate long-range phase coherence of spin by an iteration of purely dissipative quantum maps. We also demonstrate the characteristics of competition between combined coherent and dissipative non-equilibrium evolution. This opens the door for studying many-particle non-equilibrium physics and associated dynamical phase transitions with no immediate counterpart in equilibrium condensed matter systems. An error detection and reduction toolbox that facilitates the faithful quantum simulation of larger systems is developed as a first step in this direction.
Tue Dec 10th:
1. arXiv:1212.1999 [pdf, ps, other]
Study of the Shastry Sutherland Model Using Multi-scale Entanglement Renormalization Ansatz
Jie Lou, Takafumi Suzuki, Kenji Harada, Naoki Kawashima
We performed variational calculation based on the multi-scale entanglemnt renormalization ansatz, for the antiferromagnetic Heisenberg model on a Shastry Sutherland lattice (SSL). Our results show that at coupling ratio J'/J= 0.687(3), the system undergoes a quantum phase transition from the orthogonal dimer order to the plaquette valence bond solid phase, which then transits into the antiferromagnetic order above J'/J=0.75. In the presence of an external magnetic field, our calculations show clear evidences of various magnetic plateaux in systems with different coupling ratios range from 0.5 to 0.69. Our calculations are not limited to the small coupling ratio region, and we are able to show strong evidence of the presence of several supersolid phases, including ones above 1/2 and 1/3 plateaux. Such supersolid phases, which feature the coexistence of compressible superfluidity and crystalline long range order in triplet excitations, emerge at relatively large coupling ratio (J'/J>0.5). A schematic phase diagram of the SSL model in the presence of magnetic field is provided.
2. arXiv:1212.1239 [pdf, ps, other]
Entropy of Quantum States: Ambiguities
A. P. Balachandran, A. R. de Queiroz, S. Vaidya
The von Neumann entropy of a generic quantum state is not unique unless the state can be uniquely decomposed as a sum of extremal or pure states. As pointed out to us by Sorkin, this happens if the GNS representation (of the algebra of observables in some quantum state) is reducible, and some representations in the decomposition occur with non-trivial degeneracy. This non-unique entropy can occur at zero temperature. We will argue elsewhere in detail that the degeneracies in the GNS representation can be interpreted as an emergent broken gauge symmetry, and play an important role in the analysis of emergent entropy due to non-Abelian anomalies. Finally, we establish the analogue of an H-theorem for this entropy by showing that its evolution is Markovian, determined by a stochastic matrix.
3. arXiv:1212.2194 [pdf, ps, other]
Entanglement witnesses with variable number of local measurements
Wieslaw Laskowski, Marcin Markiewicz, Tomasz Paterek
We present a class of entanglement identifiers which has the following experimentally friendly feature: once the expectation value of the identifier exceeds some definite limit, we can conclude the state is entangled, even if not all measurements defining the identifier have been performed. These identifiers are in the form of sums of nonnegative functions of correlations in a quantum state, mostly squares of correlations, and we illustrate their use on various examples.
Mon Dec 10th:
1. arXiv:1212.1528 (cross-list from cond-mat.stat-mech) [pdf, ps, other]
Fidelity susceptibility of the quantum Ising model in the transverse field
Bogdan Damski
Comments: 7 pages, 3 figuresSubjects: Statistical Mechanics (cond-mat.stat-mech); Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)
We derive an exact closed-form expression for fidelity susceptibility of the quantum Ising model in the transverse field. We also establish an exact one-to-one correspondence between fidelity susceptibility in the ferromagnetic and paramagnetic phases of this model. Elegant summation formulas are obtained as a by-product of these studies.
2. arXiv:1212.1569 (cross-list from cond-mat.mes-hall) [pdf, other]
Nanomechanical read-out of a single spin
P. R. Struck, Heng Wang, Guido Burkard
Comments: 15 pages, 3 figuresSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
The spin of a single electron in a suspended carbon nanotube can be read out by using its coupling to the nano-mechanical motion of the nanotube. To show this, we consider a single electron confined within a quantum dot formed by the suspended carbon nanotube. The spin- orbit interaction induces a coupling between the spin and one of the bending modes of the suspended part of the nanotube. We calculate the response of the system to pulsed external driving of the mechanical motion using a Jaynes-Cummings model. To account for resonator damping, we solve a quantum master equation, with parameters comparable to those used in recent experiments, and show how information of the spin state of the system can be acquired by measuring its mechanical motion. The latter can be detected by observing the current through a nearby charge detector.
3. arXiv:1212.1493 [pdf, other]
The effects of strong correlations on the phase diagram and collective modes of quasi-two-dimensional dipolar fermions
Mehrtash Babadi, Brian Skinner, Michael M. Fogler, Eugene Demler
We calculate the ground state energy and phase diagram of the quasi-two-dimensional (quasi-2D) dipolar Fermi gas by variationally mapping the system to its strictly 2D counterpart for which accurate quantum Monte-Carlo (QMC) results are available. We further demonstrate the accuracy and flexibility of our variational technique by mapping the 2D dipolar Fermi gas to the 2D electron gas and showing that the resulting variational estimates closely match the QMC results. Our analysis suggests that the strongly-correlated Fermi liquid wavefunctions of different 2D Fermi systems share salient universal features. We show that inclusion of correlations significantly modify the dependence of the properties of the quasi-2D dipolar Fermi gas on the thickness of the layer compared to mean-field predictions, and calculate the frequency of its monopole oscillations in isotropic traps as an experimental demonstration of correlation effects.
Dec 3 - Dec 7, Stephan Langer
Fri Dec 7th:
1. arXiv:1212.1355 [pdf, other]
Adaptive tuning of Majorana fermions in a quantum dot chain
Ion C. Fulga, Arbel Haim, Anton R. Akhmerov, Yuval Oreg
We suggest a way to overcome the obstacles that disorder and high density of states pose to the creation of unpaired Majorana fermions in one-dimensional systems. This is achieved by splitting the system into a chain of quantum dots, which are then tuned to the conditions under which the chain can be viewed as an effective Kitaev model, so that it is in a robust topological phase with well-localized Majorana states in the outermost dots. The tuning algorithm that we develop involves controlling the gate voltages and the superconducting phases. Resonant Andreev spectroscopy allows us to make the tuning adaptive, so that each pair of dots may be tuned independently of the other. The calculated quantized zero bias conductance serves then as a natural proof of the topological nature of the tuned phase.
2. arXiv:1212.1164 [pdf, ps, other]
Thermodynamical Property of Entanglement Entropy for Excited States
Jyotirmoy Bhattacharya, Masahiro Nozaki, Tadashi Takayanagi, Tomonori Ugajin
We argue that the entanglement entropy for a very small subsystem obeys a property which is analogous to the first law of thermodynamics when we excite the system. In relativistic setups, its effective temperature is proportional to the inverse of the subsystem size. This provides a universal relationship between the energy and the amount of quantum information. We derive the results using holography and confirm them in two dimensional field theories. We will also comment on an example with negative specific heat and suggest a connection between the second law of thermodynamics and the strong subadditivity of entanglement entropy.
3. arXiv:1212.1403 [pdf, ps, other]
Phase diagram of the asymmetric Hubbard model and an entropic chromatographic method for cooling cold fermions in optical lattices
E. A. Winograd, R. Chitra, M. J. Rozenberg
We study the phase diagram of the asymmetric Hubbard model (AHM), which is characterized by different values of the hopping for the two spin projections of a fermion or equivalently, two different orbitals. This model is expected to provide a good description of a mass-imbalanced cold fermionic mixture in a 3D optical lattice. We use the dynamical mean field theory to study various physical properties of this system. In particular, we show how orbital-selective physics, observed in multi-orbital strongly correlated electron systems, can be realized in such a simple model. We find that the density distribution is a good probe of this orbital selective crossover from a Fermi liquid to a non-Fermi liquid state.
Below an ordering temperature $T_o$, which is a function of both the interaction and hopping asymmetry, the system exhibits staggered long range orbital order. Apart from the special case of the symmetric limit, i.e., Hubbard model, where there is no hopping asymmetry, this orbital order is accompanied by a true charge density wave order for all values of the hopping asymmetry. We calculate the order parameters and various physical quantities including the thermodynamics in both the ordered and disordered phases. We find that the formation of the charge density wave is signaled by an abrupt increase in the sublattice double occupancies. Finally, we propose a new method, entropic chromatography, for cooling fermionic atoms in optical lattices, by exploiting the properties of the AHM. To establish this cooling strategy on a firmer basis, we also discuss the variations in temperature induced by the adiabatic tuning of interactions and hopping parameters.
Thu Dec 6th:
1. arXiv:1212.0903 [pdf, ps, other]
Feshbach molecule formation in a Bose-Fermi mixture
Tyler D. Cumby, Ruth A. Shewmon, Ming-Guang Hu, John D. Perreault, Deborah S. Jin
We investigate magnetoassociation of ultracold fermionic Feshbach molecules in a mixture of $^{40}$K and $^{87}$Rb atoms, where we can create as many as $7\times 10^4$ $^{40}$K$^{87}$Rb molecules with a conversion efficiency as high as 45%. In the perturbative regime, we find that the conversion efficiency depends linearly on the density overlap of the two gases, with a slope that matches a parameter-free model that uses only the atom masses and the known Feshbach resonance parameters. In the saturated regime, we find that the maximum number of Feshbach molecules depends on the atoms' phase-space density. At higher temperatures, our measurements agree with a phenomenological model that successfully describes the formation of bosonic molecules from either Bose or Fermi gases. However, for quantum degenerate atom gas mixtures, we measure significantly fewer molecules than this model predicts.
2. arXiv:1212.1114 [pdf, ps, other]
Particles, holes and solitons: a matrix product state approach
Damian Draxler, Jutho Haegeman, Tobias J. Osborne, Vid Stojevic, Laurens Vanderstraeten, Frank Verstraete
We introduce a variational method for calculating dispersion relations of translation invariant (1+1)-dimensional quantum field theories. The method is based on continuous matrix product states and can be implemented efficiently. We study the Lieb-Liniger model as a benchmark where, despite criticality, excellent agreement with the exact solution is found, including, clear solitonic effects in Lieb's Type II excitation. In addition, a non-integrable model is studied where a U(1)-symmetry breaking term is added to the Lieb-Liniger Hamiltonian. For this model we find evidence of a non-trivial bound-state excitation in the dispersion relation.
3. arXiv:1212.1088 [pdf, ps, other]
Anomalous currents in driven XXZ chain with boundary twisting at weak coupling or weak driving
Vladislav Popkov, Mario Salerno
The spin 1/2 XXZ chain driven out of equilibrium by coupling with boundary reservoirs targeting perpendicular spin orientations in XY plane, is investigated. The existence of an anomaly in the nonequilibrium steady state (NESS) at the isotropic point $\Delta=1$ is demonstrated both in the weak coupling and weak driving limits. The nature of the anomaly is studied analytically by calculating exact NESS for small system sizes, and investigating steady currents. The spin current at the points $\Delta=\pm1$ has a singularity which leads to a current discontinuity when either driving or coupling vanish, and the current of energy develops a twin peak anomaly. The character of singularity is shown to depend qualitatively on whether the system size is even or odd.
Wed Dec 5th:
1. arXiv:1212.0547 [pdf, other]
BEC-BCS crossover and the mobility edge: superfluid-insulator transitions and reentrant superfluidity in disordered Fermi gases
Sarang Gopalakrishnan
A superfluid-insulator transition is known to occur in strongly disordered Fermi gases, in both the BCS and BEC regimes; here, we address the properties of this transition across the BEC-BCS crossover. We argue that the critical disorder strength at which superfluidity is lost changes non-monotonically with detuning from Feshbach resonance, and that a reentrant superfluid phase arises for detunings near the fermionic mobility edge. Our analysis of the intermediate regime is quantitatively valid for narrow resonances and near four dimensions, and provides a simple physical picture of this regime, in terms of two distinct but coexisting insulators.
2. arXiv:1212.0650 [pdf, other]
Low-Frequency Admittance as Probe of Majorana Fermions
Christophe Mora, Karyn Le Hur
We investigate the admittance of a metallic quantum RC circuit with a spinful single-channel lead or equally with two conducting spin-polarized channels, as an ideal system to probe the dynamics of Majorana fermions. We address how the two-channel Kondo physics and its emergent Majoranas arise and we determine the universal crossover function describing the Fermi-liquid to non-Fermi liquid region. Remarkably, the same universal form emerges both at weak transmission and large transmission. We find that the charge relaxation resistance strongly increases in the non-Fermi liquid realm. Our findings can be measured using current technology assuming a large cavity.
Tue Dec 4th:
1. arXiv:1212.0223 [pdf, other]
When is a bath a bath? Relaxation dynamics and thermalization in a fermionic chain
N. Sedlmayr, J. Ren, F. Gebhard, J. Sirker
We study thermalization in a one-dimensional quantum system consisting of a non-interacting fermionic chain with each site of the chain coupled to an additional bath site. Using a density matrix renormalization group algorithm we investigate the time evolution of observables in the chain after a quantum quench. For low densities we show that the intermediate time dynamics can be quantitatively described by a system of coupled equations of motion. For higher densities our numerical results show a prethermalization for local observables at intermediate times and a full thermalization to the grand canonical ensemble at long times. For the case of a weak bath-chain coupling we find, in particular, a Fermi momentum distribution in the chain in equilibrium in spite of the seemingly oversimplified bath in our model.
2. arXiv:1212.0331 [pdf]
On Locality, Growth and Transport of Entanglement
Roland Omnès
Entanglement of a macroscopic system with a microscopic one is shown to begin by a topological property of histories in the Feynman formulation of quantum mechanics. This property can also be expressed algebraically on the Schr\"odinger equation through a convenient extension of the Hilbert space formalism. Entanglement shows then properties of growth and transport, the corresponding local and temporary character of entanglement being called here "intricacy" when it occurs. When applied to the continuous interaction of a macroscopic system with a random environment, intricacy implies a "predecoherence" effect, which can generate and transport permanently incoherence within the system. The possible relevance of these results for a theory of wave function collapse is also indicated.
Mon Dec 3rd:
1. arXiv:1211.7085 [pdf, other]
Controlling Transport of Ultra-Cold Atoms in 1D Optical Lattices with Artificial Gauge Fields
Chih-Chun Chien, Massimiliano Di Ventra
We show that the recently developed optical lattices with Peierls substitution -- which can be modeled as a lattice with a complex tunneling coefficient -- may be used to induce controllable quantum transport of ultra-cold atoms. In particular, we show that by ramping up the phase of the complex tunneling coefficient in a spatially uniform fashion, a finite quasi steady-state current (QSSC) ensues from the exact dynamics of non-interacting fermions. The direction and magnitude of the current can be controlled by the overall phase difference but not the details of the ramp. The entanglement entropy does not increase when the QSSC lasts. Due to different spin statistics, condensed non-interacting bosons do not support a finite QSSC under the same setup. We also find that an approximate form of the QSSC survives when perturbative effects from interactions, weak harmonic background traps, and finite-temperature are present, which suggests that our findings should be observable with available experimental capabilities.
2. arXiv:1211.7272 [pdf, other]
Superfluidity with disorder in a quantum gas thin film
Sebastian Krinner, David Stadler, Jakob Meineke, Jean-Philippe Brantut, Tilman Esslinger
We investigate the properties of a strongly interacting, superfluid gas of 6Li2 Feshbach molecules forming a thin film confined in a quasi two-dimensional channel with a tunable random potential, creating a microscopic disorder. We measure the atomic current and extract the resistance of the film in a two-terminal configuration, and identify a superfluid state at low disorder strength, which evolves into a normal, poorly conducting state for strong disorder. The transition takes place when the chemical potential reaches the percolation threshold of the disorder. The evolution of the conduction properties contrasts with the smooth behavior of the density and compressibility across the transition, measured in-situ at equilibrium. These features suggest the emergence of a glass-like phase at strong disorder.
