Jul 2015
July 6-July 10 Haiyuan Zou, July 13-July 17 Ahmet Keles, July 20-July 24, Xuguang Yue, July 27-July 31, Max
July 24
1. arXiv:1507.06426 [pdf, ps, other]
Properties of the one-dimensional Bose-Hubbard model from a high-order perturbative expansion
Bogdan Damski, Jakub Zakrzewski
Comments: 15 pages
Subjects: Quantum Gases (cond-mat.quant-gas); Other Condensed Matter (cond-mat.other)
We employ a high-order perturbative expansion to characterize the ground state of the Mott phase of the one-dimensional Bose-Hubbard model. We compute for different integer filling factors the energy per lattice site, the two-point and density-density correlations, and expectation values of powers of the on-site number operator determining the local atom number fluctuations (variance, skewness, kurtosis). We compare these expansions to numerical simulations of the infinite-size system to determine their range of applicability. We also discuss a sum rule for the density-density correlations that can be used in both equilibrium and non-equilibrium systems.
2. arXiv:1507.06399 [pdf, ps, other]
Theoretical Analysis on Spectroscopy of Atomic Bose-Hubbard Systems
Kensuke Inaba, Makoto Yamashita
Comments: 12 pages, 4 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We provide a numerical method to calculate comprehensively the microwave and the laser spectra of ultracold bosonic atoms in optical lattices at finite temperatures. Our formulation is built up with the sum rules, up to the second order, derived from the general principle of spectroscopy. The sum rule approach allows us to discuss the physical origins of a spectral peak shift and also a peak broadening. We find that a spectral broadening of superfluid atoms can be determined from number fluctuations of atoms, while that of normal-state atoms is mainly attributed to quantum fluctuations resulting from hopping of atoms. To calculate spectra at finite temperatures, based on the sum rule approach, we provide a two-mode approximation assuming that spectra of the superfluid and normal state atoms can be calculated separately. Our method can properly deal with multi-peak structures of spectra resulting from thermal fluctuations and also coexisting of the superfluid and the normal states. By combining the two-mode approximation with a finite temperature Gutzwiller approximation, we calculate spectra at finite temperatures by considering realistic systems, and the calculated spectra show nice agreements with those in experiments.
3. arXiv:1507.06570 (cross-list from physics.atom-ph) [pdf, other]
The magic road to precision
M. S. Safronova, Z. Zuhrianda, U. I. Safronova, Charles W. Clark
Comments: 6 pages, 2 figures
Subjects: Atomic Physics (physics.atom-ph); Quantum Gases (cond-mat.quant-gas); Optics (physics.optics); Quantum Physics (quant-ph)
We predict a sequence of magic-zero wavelengths for the Sr excited 5s5p 3P0 state, and provide a general roadmap for extracting transition matrix elements using precise frequency measurements. We demonstrate that such measurements can serve as a best global benchmark of the spectroscopic accuracy that is required for the development of high-precision predictive methods. These magic-zero wavelengths are also needed for state-selective atom manipulation for implementation of quantum logic operations. We also identify five magic wavelengths of the 5s2 1S0−5s5p 3P0 Sr clock transition between 350 nm and 500 nm which can also serve as precision benchmarks.
4. arXiv:1507.05962 (cross-list from cond-mat.str-el) [pdf, ps, other]
Hyperscaling at the spin density wave quantum critical point in two dimensional metals
Aavishkar A. Patel, Philipp Strack, Subir Sachdev
Comments: 46 pages, 5 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); High Energy Physics - Theory (hep-th)
The hyperscaling property implies that spatially isotropic critical quantum states in d spatial dimensions have a specific heat which scales with temperature as Td/z, and an optical conductivity which scales with frequency as ω(d−2)/z for ω≫T, where z is the dynamic critical exponent. We examine the spin-density-wave critical fixed point of metals in d=2 found by Sur and Lee (Phys. Rev. B 91, 125136 (2015)) in an expansion in ϵ=3−d. We find that the contributions of the "hot spots" on the Fermi surface to the optical conductivity and specific heat obey hyperscaling (up to logarithms), and agree with the results of the large N analysis of the optical conductivity by Hartnoll et al. (Phys. Rev. B 84, 125115 (2011)). With a small bare velocity of the boson associated with the spin density wave order, there is an intermediate energy regime where hyperscaling is violated with d→dt, where dt=1 is the number of dimensions transverse to the Fermi surface. We also present a Boltzmann equation analysis which indicates that the hot spot contribution to the DC conductivity has the same scaling as the optical conductivity, with T replacing ω.
5. arXiv:1507.06591 (cross-list from quant-ph) [pdf, other]
Sensing Atomic Motion from the Zero Point to Room Temperature with Ultrafast Atom Interferometry
K. G. Johnson, B. Neyenhuis, J. Mizrahi, J. D. Wong-Campos, C. Monroe
Subjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph)
We sense the motion of a trapped atomic ion using a sequence of state-dependent ultrafast momentum kicks. We use this atom interferometer to characterize a nearly-pure quantum state with n=1 phonon and accurately measure thermal states ranging from near the zero-point energy to n¯∼104, with the possibility of extending at least 100 times higher in energy. The complete energy range of this method spans from the ground state to far outside of the Lamb-Dicke regime, where atomic motion is greater than the optical wavelength. These interferometric techniques are useful for characterizing ultrafast entangling gates between multiple trapped ions, and may also be used for sensing electromagnetic fields over a wide dynamic range.
July 23
1. arXiv:1507.06048 [pdf, other]
Strongly enhanced effects of Lorentz symmetry violation in entangled Yb+ ions
V. A. Dzuba, V. V. Flambaum, M. S. Safronova, S. G. Porsev, T. Pruttivarasin, M. A. Hohensee, H. Häffner
Comments: 6 pages, 3 figures
Subjects: Atomic Physics (physics.atom-ph); Optics (physics.optics); Quantum Physics (quant-ph)
Lorentz symmetry is one of the cornerstones of modern physics. However, a number of theories aiming at unifying gravity with the other fundamental interactions including string field theory suggest violation of Lorentz symmetry [1-4].
While the energy scale of such strongly Lorentz symmetry-violating physics is much higher than that currently attainable by particle accelerators, Lorentz violation may nevertheless be detectable via precision measurements at low energies [2]. Here, we carry out a systematic theoretical investigation of the sensitivity of a wide range of atomic systems to violation of local Lorentz invariance (LLI). Aim of these studies is to identify which atom shows the biggest promise to detect violation of Lorentz symmetry. We identify the Yb+ ion as an ideal system with high sensitivity as well as excellent experimental controllability. By applying quantum information inspired technology to Yb+, we expect tests of LLI violating physics in the electron-photon sector to reach levels of 10−23, five orders of magnitude more sensitive than the current best bounds [5-7]. Most importantly, the projected sensitivity of 10−23 for the Yb+ ion tests will allow for the first time to probe whether Lorentz violation is minimally suppressed at low energies for photons and electrons.
July 22
1. arXiv:1507.05937 [pdf, other]
Generation and Detection of Atomic Spin Entanglement in Optical Lattices
Han-Ning Dai, Bing Yang, Andreas Reingruber, Xiao-Fan Xu, Xiao Jiang, Yu-Ao Chen, Zhen-Sheng Yuan, Jian-Wei Pan
Comments: 12 pages, 10 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
Ultracold atoms in optical lattices offer a great promise to generate entangled states for scalable quantum information processing owing to the inherited long coherence time and controllability over a large number of particles. We report on the generation, manipulation and detection of atomic spin entanglement in an optical superlattice. Employing a spin-dependent superlattice, atomic spins in the left or right sites can be individually addressed and coherently manipulated by microwave pulses with near unitary fidelities. Spin entanglement of the two atoms in the double wells of the superlattice is generated via dynamical evolution governed by spin superexchange. By observing collisional atom loss with in-situ absorption imaging we measure spin correlations of atoms inside the double wells and obtain the lower boundary of entanglement fidelity as 0.79±0.06, and the violation of a Bell's inequality with S=2.21±0.08. The above results represent an essential step towards scalable quantum computation with ultracold atoms in optical lattices.
2. arXiv:1507.05858 (cross-list from cond-mat.stat-mech) [pdf, other]
Slow Relaxations and Non-Equilibrium Dynamics in Classical and Quantum Systems
Giulio Biroli
Comments: Lecture Notes of the 2012 Les Houches Summer School of Physics "Strongly Interacting Quantum Systems Out of Equilibrium", Oxford University Press (to appear)
Subjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el)
The aim of these lectures, given at the Les Houches Summer School of Physics "Strongly Interacting Quantum Systems Out of Equilibrium", is providing an introduction to several important and interesting facets of out of equilibrium dynamics. In recent years, there has been a boost in the research on quantum systems out of equilibrium. If fifteen years ago hard condensed matter and classical statistical physics remained rather separate research fields, now the focus on several kinds of out of equilibrium dynamics is making them closer and closer. The aim of my lectures was to present to the students the richness of this topic, insisting on the common concepts and showing that there is much to gain in considering and learning out of equilibrium dynamics as a whole research field.
3. arXiv:1507.05820 [pdf, other]
Ground Tests of Einstein's Equivalence Principle: From Lab-based to 10-m Atomic Fountains
D. Schlippert, H. Albers, L. L. Richardson, D. Nath, H. Heine, C. Meiners, É. Wodey, A. Billon, J. Hartwig, C. Schubert, N. Gaaloul, W. Ertmer, E. M. Rasel
Comments: Proceedings of the 50th Rencontres de Moriond "Gravitation: 100 years after GR", La Thuile (Italy), 21-28 March 2015
Subjects: Atomic Physics (physics.atom-ph); General Relativity and Quantum Cosmology (gr-qc)
To date, no framework combining quantum field theory and general relativity and hence unifying all four fundamental interactions, exists. Violations of the Einstein's equivalence principle (EEP), being the foundation of general relativity, may hold the key to a theory of quantum gravity. The universality of free fall (UFF), which is one of the three pillars of the EEP, has been extensively tested with classical bodies. Quantum tests of the UFF, e.g. by exploiting matter wave interferometry, allow for complementary sets of test masses, orders of magnitude larger test mass coherence lengths and investigation of spin-gravity coupling. We review our recent work towards highly sensitive matter wave tests of the UFF on ground. In this scope, the first quantum test of the UFF utilizing two different chemical elements, Rb-87 and K-39, yielding an E\"otv\"os ratio ηRb,K=(0.3±5.4)×10−7 has been performed. We assess systematic effects currently limiting the measurement at a level of parts in 108 and finally present our strategies to improve the current state-of-the-art with a test comparing the free fall of rubidium and ytterbium in a very long baseline atom interferometry setup. Here, a 10 m baseline combined with a precise control of systematic effects will enable a determination of the E\"otv\"os ratio at a level of parts in 1013 and beyond, thus reaching and overcoming the performance limit of the best classical tests.
4. arXiv:1507.05757 [pdf, other]
Non-linear Spectroscopy of Sr Atoms in an Optical Cavity for Laser Stabilization
Bjarke T. R. Christensen (1), Martin R. Henriksen (1), Stefan A. Schäffer (1), Philip G. Westergaard (2), Jun Ye (3), Murray Holland (3), Jan W. Thomsen (1) ((1) Niels Bohr Insitute, University of Copenhagen, Denmark, (2) Danish Fundamental Metrology, Kgs. Lyngby, Denmark, (3) JILA, National Institute of Standards and Technology and University of Colorado, Boulder, USA)
Comments: 7 pages, 4 figures
Subjects: Atomic Physics (physics.atom-ph)
We study the non-linear interaction of a cold sample of strontium-88 atoms coupled to a single mode of a low finesse optical cavity in the so-called bad cavity limit and investigate the implications for applications to laser stabilization. The atoms are probed on the weak inter-combination line ∣5s21S0⟩−∣5s5p3P1⟩ at 689 nm in a strongly saturated regime. Our measured observables include the atomic induced phase shift and absorption of the light field transmitted through the cavity represented by the complex cavity transmission coefficient. We demonstrate high signal-to-noise-ratio measurements of both quadratures - the cavity transmitted phase and absorption - by employing FM spectroscopy (NICE-OHMS). We also show that when FM spectroscopy is employed in connection with a cavity locked to the probe light, observables are substantially modified compared to the free space situation where no cavity is present. Furthermore, the non-linear dynamics of the phase dispersion slope is experimentally investigated and the optimal conditions for laser stabilization are established. Our experimental results are compared to state-of-the-art cavity QED theoretical calculations.
July 21
1. arXiv:1507.05279 (cross-list from cond-mat.str-el) [pdf, other]
Asymptotics of correlation functions of the Heisenberg-Ising chain in the easy-axis regime
Maxime Dugave, Frank Göhmann, Karol K. Kozlowski, Junji Suzuki
Comments: 5 Pages, 2 figures
Subjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Gases (cond-mat.quant-gas); High Energy Physics - Theory (hep-th)
We analyze the long-time large-distance asymptotics of the longitudinal correlation functions of the Heisenberg-Ising chain in the easy-axis regime. We show that in this regime the leading asymptotics of the dynamical two-point functions is entirely determined by the two-spinon contribution to their form factor expansion. Its explicit form is obtained from a saddle-point analysis of the corresponding double integral. It describes the propagation of a wave front with velocity vc1 which is found to be the maximal possible group velocity. Like in wave propagation in dispersive media the wave front is preceded by a precursor running ahead with velocityvc2. As a special case we obtain the explicit form of the asymptotics of the auto-correlation function.
2. arXiv:1507.05502 (cross-list from cond-mat.dis-nn) [pdf, other]
Role of correlations and off-diagonal terms in binary disordered one dimensional systems
Arkadiusz Kosior, Jan Major, Marcin Płodzień, Jakub Zakrzewski
Comments: 6 pages, 4 figures
Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas)
We investigate one dimensional tight binding model in the presence of a correlated binary disorder. The disorder is due to the interaction of particles with heavy immobile other species. Off-diagonal disorder is created by means of a fast periodic modulation of interspecies interaction. The method based on transfer matrix techniques allows us to calculate the energies of extended modes in the correlated binary disorder. We focus on N-mer correlations and regain known results for the case of purely diagonal disorder. For off-diagonal disorder we find resonant energies. We discuss ambiguous properties of those states and compare analytical results with numerical calculations. Separately we describe a special case of the dual random dimer model.
3. arXiv:1507.05586 (cross-list from quant-ph) [pdf, other]
Entangling two transportable neutral atoms via local spin exchange
A. M. Kaufman, B. J. Lester, M. Foss-Feig, M. L. Wall, A. M. Rey, C. A. Regal
Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Atomic Physics (physics.atom-ph)
To advance quantum information science a constant pursuit is the search for physical systems that meet the stringent requirements for creating and preserving quantum entanglement. In atomic physics, robust two-qubit entanglement is typically achieved by strong, long-range interactions in the form of Coulomb interactions between ions or dipolar interactions between Rydberg atoms. While these interactions allow fast gates, atoms subject to these interactions must overcome the associated coupling to the environment and cross-talk among qubits. Local interactions, such as those requiring significant wavefunction overlap, can alleviate these detrimental effects yet present a new challenge: To distribute entanglement, qubits must be transported, merged for interaction, and then isolated for storage and subsequent operations. Here we show how, via a mobile optical tweezer, it is possible to prepare and locally entangle two ultracold neutral atoms, and then separate them while preserving their entanglement. While ultracold neutral atom experiments have measured dynamics consistent with spin entanglement, we are now able to demonstrate two-particle coherence via application of a local gradient and parity measurements; this new entanglement-verification protocol could be applied to arbitrary spin-entangled states of spatially-separated atoms. The local entangling operation is achieved via ultracold spin-exchange interactions, and quantum tunneling is used to combine and separate atoms. Our toolset provides a framework for dynamically entangling remote qubits via local operations within a large-scale quantum register.
4. arXiv:1507.05149 [pdf, ps, other]
Momentum distribution of Cooper-pairs and strong-coupling effects in a two-dimensional Fermi gas near the Berezinskii-Kosterlitz-Thouless transition
M. Matsumoto, D. Inotani, Y. Ohashi
Comments: 7 pages, 4 figures, Proceedings of QFS2015
Subjects: Quantum Gases (cond-mat.quant-gas)
We investigate strong-coupling properties of a two-dimensional ultracold Fermi gas in the normal state. Including pairing fluctuations within the framework of a T-matrix approximation, we calculate the distribution function n(Q) of Cooper pairs in terms of the center of mass momentum Q. In the strong-coupling regime, n(Q=0) is shown to exhibit a remarkable increase with decreasing the temperature in the low temperature region, which agrees well with the recent experiment on a two-dimensional 6Li Fermi gas [M. G. Ries, {\it et. al.}, Phys. Rev. Lett. {\bf 114}, 230401 (2015)]. Our result indicates that the observed remarkable increase of the number of Cooper pairs with zero center of mass momentum can be explained without assuming the Berezinskii-Kosterlitz-Thouless (BKT) transition, when one properly includes pairing fluctuations that are enhanced by the low-dimensionality of the system. Since the BKT transition is a crucial topic in two-dimensional Fermi systems, our results would be useful for the study toward the realization of this quasi-long-range order in an ultracold Fermi gas.
5. arXiv:1507.05450 [pdf, ps, other]
Quench dynamics of dipolar fermions in a one-dimensional harmonic trap
Tobias Graß
Comments: 10 pages, 6 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We study a system of few fermions in a one-dimensional harmonic trap, and focus on the case of dipolar majority particles in contact with a single impurity. The impurity is used both for quenching the system, and for tracking the system evolution after the quench. Employing exact diagonalization, we investigate relaxation and thermalization properties. In the absence of dipolar interactions, the system is near integrability, and the dynamics remains oscillatory even on long time scales. On the other hand, repulsive as well as attractive dipolar interactions lead to quick relaxation to the diagonal ensemble average which is significantly different from corresponding thermal averages. A Wigner-shaped level spacing distribution indicates level repulsion and thus chaotic dynamical behavior due to the presence of dipolar interactions.
July 20
1. arXiv:1507.04872 [pdf, other]
Hybrid synchronization in coupled ultracold atomic gases
Haibo Qiu, Roberta Zambrini, Artur Polls, Joan Martorell, Bruno Juliá-Díaz
Comments: 11 pages, 13 figures
Subjects: Quantum Gases (cond-mat.quant-gas); Quantum Physics (quant-ph)
We study the time evolution of two coupled many-body quantum systems one of which is assumed to be Bose condensed. Specifically, we consider two ultracold atomic clouds populating each two localized single-particle states, i.e. a two-component Bosonic Josephson junction. The cold atoms cloud can retain its coherence when coupled to the condensate and displays synchronization with the latter, differing from usual entrainment. We term this effect among the ultracold and the condensed clouds as {\it hybrid synchronization}. The onset of synchronization, which we observe in the evolution of average properties of both gases when increasing their coupling, is found to be related to the many-body properties of the quantum gas, e.g. condensed fraction, quantum fluctuations of the particle number differences. We discuss the effects of different initial preparations, the influence of unequal particle numbers for the two clouds, and explore the dependence on the initial quantum state, e.g. coherent state, squeezed state and Fock state, finding essentially the same phenomenology in all cases.
2. arXiv:1507.04740 [pdf, other]
Large effective three-body interaction in a double-well optical lattice
Saurabh Paul, Eite Tiesinga
Comments: 9 pages, 7 figures
Subjects: Quantum Gases (cond-mat.quant-gas)
We study ultracold atoms in an optical lattice with two local minima per unit cell and show that the low energy states of a multi-band Bose-Hubbard (BH) Hamiltonian with only pair-wise interactions is equivalent to an effective single-band Hamiltonian with strong three-body interactions. We focus on a double-well optical lattice with a symmetric double well along the x axis and single well structure along the perpendicular directions. Tunneling and two-body interaction energies are obtained from an exact band-structure calculation and numerically-constructed Wannier functions in order to construct a BH Hamiltonian spanning the lowest two bands. Our effective Hamiltonian is constructed from the ground state of the N-atom Hamiltonian for each unit cell obtained within the subspace spanned by the Wannier functions of two lowest bands. The model includes hopping between ground states of neighboring unit cells. We show that such an effective Hamiltonian has strong three-body interactions that can be easily tuned by changing the lattice parameters. Finally, relying on numerical mean-field simulations, we show that the effective Hamiltonian is an excellent approximation of the two-band BH Hamiltonian over a wide range of lattice parameters, both in the superfluid and Mott insulator regions.
July 8
arXiv:1507.02311 [pdf, other]
Algebraic and algorithmic frameworks for optimized quantum measurements
Amine Laghaout, Ulrik L. Andersen
Subjects: Quantum Physics (quant-ph)
Von Neumann projections are the main operations by which information can be extracted from the quantum to the classical realm. They are however static processes that do not adapt to the states they measure. Advances in the field of adaptive measurement have shown that this limitation can be overcome by "wrapping" the von Neumann projectors in a higher-dimensional circuit which exploits the interplay between measurement outcomes and measurement settings. Unfortunately, the design of adaptive measurement has often been ad hoc and setup-specific. We shall here develop a unified framework for designing optimized measurements. Our approach is two-fold: The first is algebraic and formulates the problem of measurement as a simple matrix diagonalization problem. The second is algorithmic and models the optimal interaction between measurement outcomes and measurement settings as a cascaded network of conditional probabilities. Finally, we demonstrate that several figures of merit, such as Bell factors, can be improved by optimized measurements. This leads us to the promising observation that measurement detectors which---taken individually---have a low quantum efficiency can be be arranged into circuits where, collectively, the limitations of inefficiency are compensated for.
July 7
arXiv:1507.01278 [pdf, other]
Chiral spin liquids on the kagome Lattice
Krishna Kumar, Kai Sun, Eduardo Fradkin
Comments: 18 pages, 3 tables, 9 figures and 40 references
Subjects: Strongly Correlated Electrons (cond-mat.str-el)
We study the nearest neighbor XXZ Heisenberg quantum antiferromagnet on the kagome lattice. Here we consider the effects of several perturbations: a) a chirality term, b) a Dzyaloshinski-Moriya term, and c) a ring-exchange type term on the bowties of the kagome lattice, and inquire if they can suppport chiral spin liquids as ground states. The method used to study these Hamiltonians is a flux attachment transformation that maps the spins on the lattice to fermions coupled to a Chern-Simons gauge field on the kagome lattice. This transformation requires us to consistently define a Chern-Simons term on the kagome lattice. We find that the chirality term leads to a chiral spin liquid even in the absence of an uniform magnetic field, with an effective spin Hall conductance of $\sxy = \frac{1}{2}$ in the regime of XY anisotropy. The Dzyaloshinkii-Moriya term also leads a similar chiral spin liquid but only when this term is not too strong. An external magnetic field also has the possibility of giving rise to additional plateaus which also behave like chiral spin liquids in the XY regime. Finally, we consider the effects of a ring-exchange term and find that, provided its coupling constant is large enough, it may trigger a phase transition into a chiral spin liquid by the spontaneous breaking of time-reversal invariance.
