Bulletin of the American Physical Society
APS March Meeting 2015
Volume 60, Number 1
Monday–Friday, March 2–6, 2015; San Antonio, Texas
Session W36: New Developments in Cold Atom Physics |
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Sponsoring Units: DAMOP Chair: Meera Parish, Monash University Room: 211 |
Thursday, March 5, 2015 2:30PM - 2:42PM |
W36.00001: Quantifying fermionic decoherence in many-particle systems Arnab Kar, Ignacio Franco Decoherence or the loss of quantum correlations in a system arises due to the interaction of the system with its environment. Our aim is to construct measures of decoherence that are applicable to multi-electron systems and, using them, understand the relationship between electronic correlations and decoherence. Usual measures of decoherence are of limited applicability in many body systems because they are based on the $N$ particle density matrix which is generally not available. Here, we propose a hierarchy of measures of decoherence called distilled purities that are based on the hierarchy of $r$-particle reduced density matrices [1-2]. Given a single particle basis, these measures can be used to succinctly capture relevant coherences and interpret decoherence dynamics in driven and non-driven many body systems. The distilled purity measures will be exemplified using the dynamics of the Su-Schrieffer-Heeger Hamiltonian for trans-polyacetylene. The advantages and limitations of these distilled purity measures will also be discussed. \\[4pt] [1] I. Franco, H. Appel, J. Chem. Phys. 139, 094109 (2013)\\[0pt] [2] I. Franco and P. Brumer, J. Chem. Phys. 136, 144501 (2012) [Preview Abstract] |
Thursday, March 5, 2015 2:42PM - 2:54PM |
W36.00002: High Resolution Neutral Atom Microscope Igal Bucay, Rodrigo Castillo-Garza, Georgios Stratis, Mark Raizen We are developing a high resolution neutral atom microscope based on metastable atom electron spectroscopy (MAES). When a metastable atom of a noble gas is near a solid, a surface electron will tunnel to an empty energy level of the metastable atom, thereby ejecting the excited electron from the atom. The emitted electrons carry information regarding the local topography and electronic, magnetic, and chemical structures of most hard materials. Furthermore, using a chromatic aberration corrected magnetic hexapole lens we expect to attain a spatial resolution below 10 nm. We will use this microscope to investigate how local phenomena can give rise to macroscopic effects in materials that cannot be probed using a scanning tunneling microscope, namely insulating transition metal oxides. [Preview Abstract] |
Thursday, March 5, 2015 2:54PM - 3:06PM |
W36.00003: Feshbach Modulation Spectroscopy James Freericks, Andreas Dirks, Hulikal Krishnamurthy, Karlis Mikelsons Feshbach resonances are often swept through to bind atoms into weakly bound molecules. Here we propose to examine the behavior of systems close to a Feshbach resonance when we modulate the bias magnetic field as a function of time, creating a modulated scattering length. On an optical lattice, this system undergoes rich physical transformations which involve both molecule formation and the hopping of molecules on the lattice and thus goes beyond a single-band Hubbard model description. While experiments have already studied some of this phenomena, especially resonance effects on the binding of molecules, we feel that this system is likely to have interesting physical behavior when on an optical lattice. We propose to probe the behavior with a harmonic modulation of the magnetic field and thus the scattering length across the Feshbach resonance as a generalization of lattice-depth modulation spectroscopy. In the regime in which the single-band Hubbard model is still valid, we provide simulation data for this type of spectroscopy which behaves somewhat differently from conventional modulation spectroscopy (the hopping is not modulated, just the interaction strength). [Preview Abstract] |
Thursday, March 5, 2015 3:06PM - 3:18PM |
W36.00004: Quantum Melting in a Polariton Lattice Alexander Edelman, Peter Littlewood We study a generalized Dicke model of lattice polaritons, with a pair-potential interaction between excited states of the spin component, in the functional integral formalism. Even considering only zero-temperature equilibrium effects with a uniform photon field, there is a rich phase diagram as a function of light-matter coupling, which includes spatially ordered and superfluid phases. Depending sensitively on the form of the potential, the interaction may induce an instability in the sound mode of the polariton condensate, or destroy the condensate altogether. Zero-temperature fluctuations may likewise melt the spatially ordered phases. We consider implications for cold-atom experiments with tunable interactions, as well as interacting exciton-polaritons accessible in the solid state. [Preview Abstract] |
Thursday, March 5, 2015 3:18PM - 3:30PM |
W36.00005: Quantum fluctuations and gapped Goldstone modes in spinor Bose-Einstein condensates Aron Beekman The classical Heisenberg ferromagnet is an exact eigenstate of the quantum Hamiltonian and therefore has no quantum fluctuations. Furthermore it has a reduced number of Goldstone modes, an order parameter that is itself a symmetry generator, is a highest-weight state for the spin algebra, and has no tower of states of vanishing energy. We derive the connection between all these properties and provide general criteria for their presence in other spontaneously-broken symmetry states. The phletora of groundstates in spinor Bose-Einstein condensates is an ideal testing ground for these predictions. In particular the phases with non-maximal polarization (e.g. the F-phase in spin-3 condensates) have an additional gapped mode that is a partner to the quadratically dispersing Goldstone mode, as compared to the maximally polarized, ferromagnetic phase. Furthermore there is a fundamental limit to the coherence time of superpositions in the non-maximally polarized state, which should manifest itself for small-size systems. [Preview Abstract] |
Thursday, March 5, 2015 3:30PM - 3:42PM |
W36.00006: Emergent dual space-time geometry for free fermions Ching Hua Lee, Xiao-Liang Qi The theme of holography has attracted great interest among high energy and condensed matter physicists alike. It involves describing a 'boundary' system in terms of a 'bulk' system in a space one dimension higher, with the emergent direction representing energy scale. We propose a simple exact holographic mapping for lattice systems based on wavelet bases, which naturally entail an emergent dimension representing scale. The system in the new basis is identified as the bulk, whose correlation functions can be interpreted as that of a massive field in curved spacetime. Despite the simplicity of our approach, we obtain in the long-wavelength limit geometries that are consistent with those expected from the Ryu-Takanayagi formula, i.e. AdS space for critical zero-temperature systems, a paradigmatic example of the AdS-CFT correspondence. At nonzero temperature, we obtain the BTZ and Lifshitz black holes for linear and nonlinear critical band touchings respectively, as we analytically verify up to the subleading logarithmic correction. Our results remain true in any number of dimensions, under generic local wavelet bases. [Preview Abstract] |
Thursday, March 5, 2015 3:42PM - 3:54PM |
W36.00007: Supersymmetry in quantum optics and in spin-orbit coupled systems Michael Tomka, Mikhail Pletyukhov, Vladimir Gritsev Light-matter interaction is naturally described by coupled bosonic and fermionic subsystems. This suggests that a certain Bose-Fermi duality is naturally present in the fundamental quantum mechanical description of photons interacting with atoms. We reveal submanifolds in parameter space of a basic light-matter interacting system where this duality is promoted to a supersymmetry (SUSY) which remains unbroken. We show that SUSY is robust with respect to decoherence and dissipation. In particular, the stationary density matrix at the supersymmetric lines in parameter space has a degenerate subspace. The dimension of this subspace is given by the Witten index and thus topologically protected. As a consequence of this SUSY, dissipative dynamics at the supersymmetric lines is constrained by an additional conserved quantity which translates some part of information about an initial state into the stationary state subspace. We also demonstrate a robustness of this additional conserved quantity away from the supersymmetric lines. [Preview Abstract] |
Thursday, March 5, 2015 3:54PM - 4:06PM |
W36.00008: Magnetization and Transport Properties for Particles in Spin Textures Timothy McCormick, Nandini Trivedi We use exact-diagonalization and Monte Carlo (ED$+$MC) to calculate the magnetization M(T) and the spin polarization P(T) for a charged particle moving in a variety of ferromagnetic, spiral and chiral spin textures. We derive an effective spin Hamiltonian by integrating out charged degrees of freedom and compare its magnetization with that of the full Hamiltonian. We then calculate transport properties such as the dynamical conductivity $\backslash $sigma($\backslash $omega) and the anomalous Hall conductivity using the Chern number. [Preview Abstract] |
Thursday, March 5, 2015 4:06PM - 4:18PM |
W36.00009: Emergent Magnetic Monopole Charges in a Two Qubit System Tiago Grangeiro Souza Barbosa Lima, Michael Kolodrubetz, Anatoli Polkovnikov The topology of two coupled qubits has recently been explored using dynamical measurements of their Berry curvature $\vec{F}$. Its integral gives the topologically invariant Chern number, which natural maps to the presence of magnetic monopole charges in parameter space. We suggest a method for measuring the magnetic monopole charge density and detail its motion as external parameters are varied. Using Maxwell's equations with the addition of magnetic monopoles, we obtain the effective charge density and the current density as a function of the parameters. We show how particular choices for parameters give rise to peculiar motion of the monopole charges, which can go from isolated charges to continuous charge distributions like rings and surfaces by properly changing the system's symmetries through its parameters. Finally, we probe the interesting but as yet unexplored consequences on $\nabla \times \vec{F}$ as said changes are made. [Preview Abstract] |
Thursday, March 5, 2015 4:18PM - 4:30PM |
W36.00010: Entropy flow in quantum heat engines Mohammad Ansari, Yuli Nazarov We evaluate Shannon and Renyi entropy flows from generic quantum heat engines (QHE) to a weakly-coupled probe environment kept in thermal equilibrium. We show the flows are determined by two quantities: heat flow and fictitious dissipation that manifest the quantum coherence in the engine. Our theory leads to novel physics in quantum heat engines. [Preview Abstract] |
Thursday, March 5, 2015 4:30PM - 4:42PM |
W36.00011: Dynamics of Noisy Quantum Systems in the Heisenberg Picture: Application to the Stability of Fractional Charge Armin Rahmani Based on the Heisenberg-picture analog of the master equation, we develop a method for computing the exact time dependence of noise-averaged observables for (generally interacting) fermionic systems with noisy hopping processes. Our results provide access to a long-time limit, which is not amenable to numerical simulations. As a simple example, we examine the fate of the fractional charge in a noisy dimerized lattice with a domain wall (relevant to cold-atom emulations of polyacetylene). We find that the fractional charge remains robust against noisy hopping processes between different sublattices, while it becomes unstable to fluctuations in hopping on the same sublattice. [Preview Abstract] |
Thursday, March 5, 2015 4:42PM - 4:54PM |
W36.00012: Fermions in a harmonic trap with spin-imbalanced filling Denis Morath, Stefan A. Soeffing, Sebastian Eggert In recent experiments with ultra-cold fermions it was possible to prepare states with imbalanced pseudo-spin fillings, analogous to electrons in quantum dots. This offers the opportunity to make controlled studies of the influence of finite interactions, spin filling and temperature on the density of confined fermions, We now consider the situation in a one-dimensional trap theoretically and with numerical quantum simulations (quantum Monte Carlo and DMRG). Already for three particles in a trap there is a surprising alignment of spin up an down particles with a rather dramatic effect of the temperature. Naively an antiferromagnetic correlation between the spin species should be expected for repulsive interactions, i.e.~density maxima of spin-up should correlate in space with spin-down minima and vice versa. However, already very low finite temperatures can induce {\it ferromagnetic} correlations. Based on the analysis of few particle situations and symmetry considerations we can also explain the behaviour of many particle systems. [Preview Abstract] |
Thursday, March 5, 2015 4:54PM - 5:06PM |
W36.00013: Many-Body Transition in a Spin-Orbit Coupled Bose-Einstein Condensate Jeffrey T.F. Poon, Xiong-Jun Liu In quantum mechanics, a resonant Rabi oscillation can occur between two degenerate single-particle states when such two states are subject to external perturbations. This phenomenon can be qualitatively different in the interacting regime. In this work, we study a spin-orbit coupled Bose-Einstein condensate with degenerate many-body states and examine the transitions between such states. We find that due to the particle-particle interactions the many-body transitions between such degenerate states are completely different from the physics in single-particle systems. Both the numerical and analytic results will be discussed. [Preview Abstract] |
Thursday, March 5, 2015 5:06PM - 5:18PM |
W36.00014: Landau-Zener transitions in a two-level system that is coupled to a finite-temperature harmonic oscillator Sahel Ashhab The Landau-Zener (LZ) problem is a standard paradigm for studying energy transfer and adiabatic passage protocols. We consider the LZ problem for a two level system when this system interacts with one harmonic oscillator mode that is initially set to a finite-temperature thermal equilibrium state. The oscillator could represent an external mode that is strongly coupled to the system, e.g. an ionic oscillation mode in a molecule, or it could represent a prototypical uncontrolled environment. We analyze the system's occupation probabilities at the final time in a number of different regimes, varying the system and oscillator frequencies, their coupling strength and the temperature. In particular we find some surprising non-monotonic dependence on the coupling strength and temperature. [Preview Abstract] |
Thursday, March 5, 2015 5:18PM - 5:30PM |
W36.00015: Effects of nonmagnetic impurities on BCS-BEC crossover in atomic Fermi gases Yanming Che, Qijin Chen We present a systematic investigation of the effects of nonmagnetic impurities on the $s$-wave BCS-BEC crossover within a pairing fluctuation theory. Both the pairing $T$-matrix and the impurity scattering $T$-matrix are treated self-consistently at the same time, in the context of ultracold atomic Fermi gases. While the system is less sensitive to impurity scattering in the Born limit, in the strong impurity scattering limit, both the frequency and the gap function are highly renormalized, leading to significant suppression of the superfluid $T_c$. In the BCS regime, the superfluidity may be readily destroyed by the impurity, leading to an effective power law dependence of $T_c$ as a function of pairing strength. In comparison, $T_c$ and (pseudo)gaps in the unitary and BEC regimes are relatively more robust. In either cases, the $s$-wave pairing is less sensitive to impurity than its $d$-wave counterpart. Calculations of superfluid density will also be presented. References: Q.J. Chen and J.R. Schrieffer, Phys. Rev. B 66, 014512 (2002). [Preview Abstract] |
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