Highlights

Highlights

Characteristics of Two-Dimensional Quantum Turbulence in a Compressible Superfluid

preprint

Under suitable forcing a fluid exhibits turbulence, with characteristics strongly affected by the fluid's confining geometry. Here we study two-dimensional quantum turbulence in a highly oblate Bose-Einstein condensate in an annular trap. As a compressible quantum fluid, this system affords a rich phenomenology, allowing coupling between vortex and acoustic energy. Small-scale stirring generates an experimentally observed disordered vortex distribution that evolves into large-scale flow in the form of a persistent current. Numerical simulation of the experiment reveals additional characteristics of two-dimensional quantum turbulence: spontaneous clustering of same-circulation vortices, and an incompressible energy spectrum with -5/3 power-law dependence for low wave numbers and -3 power-law dependence for high wave numbers.
April 2012.

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Hot spinor gases

preprint

We formulate a self-consistent Hartree-Fock theory for a spin-1 Bose gas at finite temperature and apply it to characterizing the phase diagram. We find that spin coherence between thermal atoms in different magnetic sub-levels develops via coherent collisions with the condensed atoms, and is a crucial factor in determining the phase diagram. We develop analytical expressions to characterize the interaction and temperature dependent shifts of the phase boundaries..
February 2012.

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Quantum tunneling of a vortex between two pinning potentials

Phys. Rev. Lett. 108, 015301 (2012)

A vortex can tunnel between two pinning potentials in an atomic Bose-Einstein condensate on a time scale of the order of 1s under typical experimental conditions. This makes it possible to detect the tunneling experimentally. We calculate the tunneling rate by phenomenologically treating vortices as point-like charged particles moving in an inhomogeneous magnetic field. The obtained results are in close agreement with numerical simulations based on the stochastic c-field theory.
December 2011.

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Suppression of Kelvon-induced decay of quantized vortices in oblate Bose-Einstein condensates

Phys. Rev. A 84, 023637 (2011)

We study the Kelvin mode excitations on a vortex line in a three-dimensional trapped Bose-Einstein condensate at finite temperature. Our stochastic Gross-Pitaevskii simulations show that the activation of these modes can be suppressed by tightening the confinement along the direction of the vortex line, leading to a strong suppression in the vortex decay rate as the system enters a regime of two-dimensional vortex dynamics. As the system approaches the condensation transition temperature, we find that the vortex decay rate is strongly sensitive to dimensionality and temperature, observing a large enhancement for quasi-two-dimensional traps. Three-dimensional simulations of the recent vortex dipole decay experiment of Neely et al. [ Phys. Rev. Lett. 104 160401 (2010)] confirm two-dimensional vortex dynamics and predict a dipole lifetime consistent with experimental observations and suppression of Kelvon-induced vortex decay in highly oblate condensates.
Aug 2011.

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Direct simulation Monte Carlo method for cold-atom dynamics: Classical Boltzmann equation in the quantum collision regime

Phys. Rev. A 84, 023612 (2011)

We develop a direct simulation Monte Carlo method for simulating highly nonequilibrium dynamics of nondegenerate ultracold gases. We show that our method can simulate the high-energy collision of two thermal clouds in the regime observed in experiments [Thomas et al. Phys. Rev. Lett. 93, 173201 (2004)], which requires the inclusion of beyond s-wave scattering. We also consider the long-time dynamics of this system, demonstrating that this would be a practical experimental scenario for testing the Boltzmann equation and studying rethermalization.
Aug 2011.

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Finite-temperature stability of a trapped dipolar Bose gas

Phys. Rev. A 83, 061602(R) (2011)

We calculate the stability diagram for a trapped normal Bose gas with dipole-dipole interactions. Our study characterizes the roles of trap geometry, temperature, and short-range interactions on the stability. We predict a robust double instability feature in oblate trapping geometries arising from the interplay of thermal gas saturation and the anisotropy of the interaction. Our results are relevant to current experiments with polar molecules and will be useful in developing strategies to obtain a polar molecule Bose-Einstein condensate.
Jun 2011.

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Thermally induced coherence in a Mott insulator of bosonic atoms

Phys. Rev. A 83, 021601(R) (2011)

Naively, one may think that increasing temperature causes quantum coherence to decrease. Using finite-temperature perturbation theory and exact calculations for the strongly correlated bosonic Mott insulating state, we show a practical counterexample that can be explored in optical lattice experiments: the short-range coherence of the Mott insulating phase can increase substantially with increasing temperature. We demonstrate that this phenomenon originates from thermally produced defects that can tunnel with ease. Since the near-zero temperature coherence properties have been measured with high precision, we expect these results to be verifiable in current experiments.
Feb 2011.

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Analysis of a continuous-variable quadripartite cluster state from a single optical parametric oscillator

Phys. Rev. A 82, 053826 (2010)

We examine the feasibility of generating continuous-variable multipartite entanglement in an intracavity concurrent downconversion scheme that has been proposed for the generation of cluster states by Menicucci et al. [Phys. Rev. Lett. 101, 130501 (2008)]. By calculating optimized versions of the van Loock-Furusawa correlations we demonstrate genuine quadripartite entanglement and investigate the degree of entanglement present. Above the oscillation threshold the basic cluster state geometry under consideration suffers from phase diffusion. We alleviate this problem by incorporating a small injected signal into our analysis. Finally, we investigate squeezed joint operators. While the squeezed joint operators approach zero in the undepleted regime, we find that this is not the case when we consider the full interaction Hamiltonian and the presence of a cavity. In fact, we find that the decay of these operators is minimal in a cavity, and even depletion alone inhibits cluster state formation.
Nov 2010.

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Geometric scale invariance as a route to macroscopic degeneracy: Loading a toroidal trap with a Bose or Fermi gas

Phys. Rev. A 82, 013626 (2010)

An easily scalable toroidal geometry presents an opportunity for creating large-scale persistent currents in Bose-Einstein condensates, for studies of the Kibble-Zurek mechanism, and for investigations of toroidally trapped degenerate Fermi gases. We consider in detail the process of isentropic loading of a Bose or Fermi gas from a harmonic trap into the scale-invariant toroidal regime that exhibits a high degree of system invariance when increasing the radius of the toroid. The heating involved in loading a Bose gas is evaluated analytically and numerically, both above and below the critical temperature. Our numerical calculations treat interactions within the Hartree-Fock-Bogoliubov-Popov theory. Minimal change in degeneracy is observed over a wide range of initial temperatures, and a regime of cooling is identified. The scale-invariant property is further investigated analytically by studying the density of states of the system, revealing the robust nature of scale invariance in this trap, for both bosons and fermions. We give analytical results for a Thomas-Fermi treatment. We calculate the heating due to loading a spin-polarized Fermi gas and compare with analytical results for high- and low-temperature regimes. The Fermi gas is subjected to irreducible heating during loading, caused by the loss of one degree of freedom for thermalization.
Jul 2010.

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Observation of vortex dipoles in an oblate Bose-Einstein condensate

Physical Review Letters 104, 160401 (2010)

A collaborative team from Otago, Arizona, and Queensland have directly observed and studied vortex dipoles in a dilute gas BEC for the first time. Previous studies observed indirect evidence for the breakdown of superfluidity at a critical velocity (related to the Landau critical velocty) while dragging an obstacle through a BEC. The experimental team at Arizona was able to directly observe vortex dipole formation and dynamics above a critical sweep velocity -a velocity that agrees well with earlier theory, and numerical simulations of the experiment.

Surprisingly, for faster sweep velocities multiple dipoles form, and aggregate into giant macro-dipoles.

For an overview, see the Physics Viewpoint article featuring this work.

May 2010.

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Vortex decay in a warm Bose-Einstein condensate

Phys. Rev. A 81, 023630 (2010)

The decay of a vortex in a non-rotating Bose-Einstien condensate has been used as a sensitive test of theories of non-equilibrium dynamics at finite temperature in a recent article by honours student Sam Rooney, Ashton Bradley, and Blair Blakie. The Stochastic theory of reservoir interactions was found to provide the most complete description of vortex decay, predicting significantly shorter vortex lifetimes than theories that neglect either damping or noise.

The approach used in this work is also distinguished by having no fitting parameters, and hence may be of quantitative value for future experiments.

Feb 2010.

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C-field methods for Bose gases

Advances in Physics 57, 363 (2008).

The truncated Wigner phase space method has been extensively developed and applied to dilute Bose gases in recent years. In this approach the quantum field evolution can be represented using equations of motion of a similar form to the Gross–Pitaevskii equation but with stochastic modifications that include quantum effects in a controlled degree of approximation. These techniques provide a practical quantitative description of both equilibrium and dynamical properties of Bose gas systems. This review starts with an overview of formalism and gives a survey of some of the recent work on modeling trapped Bose gases. Applications include zero, finite temperature, and critical regimes, low dimensional systems, and rotating systems.

Feb 2010.

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Spontaneous vortices in the formation of Bose-Einstein condensates

Nature 455, 948-951 (2008).

Despite long standing predictions by W. H. Zurek and collaborators that vortices should occur spontaneously during the Bose-Einstein condensation transition, they have evaded experimental detection, until now.

The experimental observations of B. P. Anderson's group (Arizona) have been quantitatively explained using the Stochastic Projected Gross-Pitaevskii equation (SPGPE) of C. W. Gardiner (Otago) and M. J. Davis (Queensland).

For more about spontaneous vortices, see the Nature commentary by Professor Kris Helmerson.

This work on spontaneous vortices also featured in a radio interview of Ashton Bradley and Crispin Gardiner by Veronika Meduna on Our Changing World: Quantum Whirlpools.
Feb 2010.

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Numerical methods for the PGPE

Physical Review E 78, 026704 (2008)

The secrets of our long-developed numerical methods for simulating the projected Gross-Pitaevskii equation in a harmonic trap are revealed in this work. Evidence for thermalization in the trapped system is also presented.
Feb 2010.

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Theory of correlations between bosons released from an optical lattice

Physical Review A 78, 013627 (2008)

Ms. Emese Toth, in collaboration with Dr. A.-M. Rey (ITAMP/JILA), has developed a comprehensive theory for correlations between bosons released from an optical lattice.
Feb 2010.

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Tests for superfluidity of trapped quasi2D Bose gases

Physical Review A 77, 023618 (2008)

In collaboration with Drs. Tapio Simula and Matthew Davis, we have published the results of a study examining signatures for the onset of superfluidity in a quasi-2D Bose gas, expected to happen at the Berezinskii-Kosterlitz-Thouless transition. Our results show that scissor mode oscillations can be used to reveal the reduction in rotational inertia arising from the appearance of a superfluid component.
Feb 2010.

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Theory for finite-T correlations in a trapped interacting Bose gas

Physical Review A 77, 023602 (2008)

Ms Alice Bezett has completed a formalism for calculating position and momentum space correlation functions appropriate to real experimental parameter regimes. Based on a classical field method, this formalism is valid in the critical region and should be applicable to current and future experiments.
Feb 2010.

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Theory-experiment study of lattice dynamical instabilities

Physical Review A 77, 012712 (2008)

In collaboration with the experimental group of Prof. Andrew Wilson and the theory group of Dr. Matthew Davis we have completed a detailed study of condensate instabilities when prepared near the Brillouin zone boundary using an optical lattice. Large scale quantum simulations with the truncated Wigner method reveal the important role played by vacuum and thermal fluctuations.


Feb 2010.

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Theory of Fermi gas loading in optical lattice

Physical Review A 75, 063609 (2007)

A quantitative theory has been developed for the thermal properties of a degenerate Fermi gas loaded from a harmonic trap into a combined harmonic optical lattice potential. Our results show that this commonly used experimental procedure typically causes a factor of 2 degradation in the degeneracy temperature. We show how the use of excited bands can overcome this limitation.

Feb 2010.

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Revealing the Bose Glass Phase

Physical Review A 76, 011602(R) (2007)

In collaboration with Dr Buonsante and colleagues in Torino a meanfield decoupling approach is used to model the properties of a Bose gas in a deep optical lattice with disorder. We characterize the phase diagram of this system at zero and finite temperatures and elucidate the regimes where the Bose Glass phase appears. This work appears in Physical Review A

Feb 2010.

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Spectroscopy of Mott Insulator States

New Journal of Physics 8, 157 (2006

In this work I propose using a novel Raman spectroscopy scheme for probing the Mott-insulating state of a Bose gas in a deep optical lattice. By developing a new linear response analysis that retains correlations between the system and excited particles, I demonstrate that the excitation defects give rise to excitonic-like states in the spectrum of this system. This work appears in New Journal of Physics. Note: recent results by Wolfgang Ketterle's group reported in Science appear to be in good agreement with our theory.

Feb 2010.

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Georgia Tech Experiment Verifies Vortex Bragg Scattering Theory

Physical Review A 73, 041605(R) (2006)

In a 2001 Physical Review Letter with Prof. Rob Ballagh, we proposed the idea that Bragg Scattering could be used as a spatially sensitive mechanism for detecting vortices in Bose Einstein condensates. Recent experiments by Chandra Raman's group at Georgia Tech have confirmed these results. More can be seen at the Raman Group homepage.

Feb 2010.

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Thermal Vortex-Pair Creation in 2D Bose-Einstein Condensates

Physical Review Letters, 73, 023604 (2006)

Work done in collaboration with Dr Tapio Simula used classical field techniques to explore the superfluid phase diagram of a trapped 2D Bose gas. These theoretical results are in good agreement with recent experiments by the ENS group of Jean Dalibard. This work appears in Physical Review Letters and additional results and movies from this work are available here.

Feb 2010.