arXiv subjects

C. S. Adams

Publications and source records attributed to C. S. Adams.

At least 73 records · Page 4Linked to original sources

arXiv2006

Bright Matter-Wave Soliton Collisions in a Harmonic Trap: Regular and Chaotic Dynamics

Collisions between bright solitary waves in the 1D Gross-Pitaevskii equation with a harmonic potential, which models a trapped atomic Bose-Einstein condensate, are investigated theoretically. A particle analogy for the solitary waves is formulated, which is shown to be integrable for a two-particle system. The extension to three particles is shown to support chaotic regimes. Good agreement is found between the particle model and simulations of the full wave dynamics, suggesting that the dynamics can be described in terms of solitons both in regular and chaotic regimes, thus presenting a paradigm for chaos in wave-mechanics.

cond-mat.other↗

arXiv2006

Polarization spectroscopy in rubidium and cesium

We develop a theoretical treatment of polarization spectroscopy and use it to make predictions about the general form of polarization spectra in the alkali atoms. Using our model, we generate theoretical spectra for the D2 transitions in 87Rb, 85Rb, and 133Cs. Experiments demonstrate that the model accurately reproduces spectra of transitions from the upper hyperfine level of the ground state only. Among these, the closed transition F -> F' = F + 1 dominates, with a steep gradient through line center ideally suited for use as a reference in laser locking.

physics.atom-ph↗

arXiv2006

Collisions between solitary waves of three-dimensional Bose-Einstein condensates

We study bright solitary waves of three dimensional trapped Bose-Einstein condensates and their collisions. For a single solitary wave, in addition to an upper critical number, we also find a {\em lower} cut-off, below which no stable state can be found. Collisions between solitary waves can be elastic, inelastic with either reduced or increased outgoing speed, or completely unstable due to a collapse instability. A $π$-phase difference between the waves promotes elastic collisions, and gives excellent agreement with recent experimental results over long timescales.

cond-mat.other↗

arXiv2005

Response of an atomic Bose-Einstein condensate to a rotating elliptical trap

We investigate numerically the response of an atomic Bose-Einstein condensate to a weakly-elliptical rotating trap over a large range of rotation frequencies. We analyse the quadrupolar shape oscillation excited by rotation, and discriminate between its stable and unstable regimes. In the latter case, where a vortex lattice forms, we compare with experimental observations and find good agreement. By examining the role of thermal atoms in the process, we infer that the process is temperature-independent, and show how terminating the rotation gives control over the number of vortices in the lattice. We also study the case of critical rotation at the trap frequency, and observe large centre-of-mass oscillations of the condensate.

cond-mat.other↗

arXiv2005

Spatially selective loading of an optical lattice by light-shift engineering using an auxiliary laser field

We report on a method of light-shift engineering where an auxiliary laser is used to tune the atomic transition frequency. The technique is used to selectively load a specific region of an optical lattice. The results are explained by calculating the differential light-shift of each hyperfine state. We conclude that the remarkable spatial selectivity of light-shift engineering using an auxiliary laser provides a powerful technique to prepare ultra-cold trapped atoms for experiments on quantum gases and quantum information processing.

physics.atom-ph↗

arXiv2005

Emergence and decay of turbulence in stirred atomic Bose-Einstein condensates

We show that a `weak' elliptical deformation of an atomic Bose-Einstein condensate rotating at close to the quadrupole instability frequency leads to turbulence with a Kolmogorov energy spectrum. The turbulent state is produced by energy transfer to condensate fragments that are ejected by the quadrupole instability. This energy transfer is driven by breaking the two-fold rotational symmetry of the condensate. Subsequently, vortex-sound interactions damp the turbulent state leading to the crystalization of a vortex lattice.

cond-mat.other↗

arXiv2004

Analogies between dark solitons in atomic Bose-Einstein condensates and optical systems

Dark solitons have been observed in optical systems (optical fibres, dielectric guides and bulk media), and, more recently, in harmonically confined atomic Bose-Einstein condensates. This paper presents an overview of some of the common features and analogies experienced by these two intrinsically nonlinear systems, with emphasis on the stability of dark solitons in such systems and their decay via emission of radiation. The closely related issue of vortex dynamics in such systems is also briefly discussed.

cond-mat.other↗

arXiv2004

Parametric Driving of Dark Solitons in Atomic Bose-Einstein Condensates

A dark soliton oscillating in an elongated harmonically-confined atomic Bose-Einstein condensate continuously exchanges energy with the sound field. Periodic optical `paddles' are employed to controllably enhance the sound density and transfer energy to the soliton, analogous to parametric driving. In the absence of damping, the amplitude of the soliton oscillations can be dramatically reduced, whereas with damping, a driven soliton equilibrates as a stable dark soliton with lower energy, thereby extending the soliton lifetime up to the lifetime of the condensate.

cond-mat.other↗

arXiv2004

Tunnelling induced collapse of an atomic Bose-Einstein condensate in a double-well potential

The stability of a low temperature Bose-Einstein condensate with attractive interactions in one and three dimensional double-well potentials is discussed. In particular, the tunnelling dynamics of a condensate under the influence of a time-dependent potential gradient is investigated. The condensate is shown to collapse at a critical potential gradient which corresponds to a critical number of atoms in one of the two wells. The sensitivity of this tunnelling induced collapse could provide a useful tool in the study of condensates with attractive interactions.

cond-mat.other↗

arXiv2004

Decay of quantised vorticity by sound emission

It is thought that in a quantum fluid sound generation is the ultimate sink of turbulent kinetic energy in the absence of any other dissipation mechanism near absolute zero. We show that a suitably trapped Bose-Einstein condensate provides a model system to study the sound emitted by accelerating vortices in a controlled way.

cond-mat.other↗

arXiv2004

Josephson tunnelling of a phase-imprinted Bose-Einstein condensate in a time-dependent double-well potential

This paper discusses the feasibility of experimental control of the flow direction of atomic Bose-Einstein condensates in a double-well potential using phase-imprinting. The flow is induced by the application of a time-dependent potential gradient, providing a clear signature of macroscopic quantum tunneling in atomic condensates. By studying both initial state preparation and subsequent tunneling dynamics we find the parameters to optimise the phase induced Josephson current. We find that the effect is largest for condensates up to a few thousand atoms, and is only weakly-dependent on trap geometry.

cond-mat.other↗

arXiv2003

Controlled vortex-sound interactions in atomic Bose-Einstein condensates

The low temperature dynamics of a vortex in a trapped quasi-two-dimensional Bose-Einstein condensate are studied quantitatively. Precession of an off-centred vortex in a dimple trap, embedded in a weaker harmonic trap, leads to the emission of sound in a dipolar radiation pattern. Sound emission and reabsorption can be controlled by varying the depth of the dimple. In a shallow dimple, the power emitted is proportional to the vortex acceleration squared over the precession frequency, whereas for a deep dimple, periodic sound reabsorption stabilises the vortex against radiation-induced decay.

cond-mat↗

arXiv2003

Josephson Current Counterflow in Atomic Bose-Einstein Condensates

Josephson weak links in superconductors can be engineered such that the phase difference across the junction is $π$ ($π$-junction),leading to the reversal of the current. The conditions under which the analogous effect of supercurrent counterflow can be achieved in a double-well atomic Bose-Einstein condensate are investigated. It is shown that this effect is observable for condensates up to a few thousand atoms, which are initially prepared in an anti-symmetric '$π$-state' and subsequently subjected to a uniformly increasing magnetic field gradient. This effect is found to be only weakly-dependent on trap geometry, and can be observed in both attractive and repulsive condensates.

cond-mat↗

arXiv2003

Dynamical instability of a dark soliton in a quasi-one-dimensional Bose-Einstein condensate perturbed by an optical lattice

The motion of a dark soliton is investigated in a one-dimensional dilute Bose-Einstein condensate confined in a harmonic trap and an optical lattice. The harmonic trap induces a dynamical instability of the soliton, culminating in sound emission. The presence of the optical lattice enhances the instability, and in addition, dephases the emitted sound waves, thus preventing stabilisation of the soliton by sound reabsorption. This instability can be probed experimentally by monitoring the soliton oscillations under various lattice configurations, which can be realised by changing the intensity and angle between the laser beams that form the lattice. For short enough times, such that the emitted sound does not reinteract with the soliton, the power emitted by the soliton is found to be proportional to the square of the local soliton acceleration, which is in turn proportional to the deformation of the soliton profile.

cond-mat↗

arXiv2003

Deformation of dark solitons in inhomogeneous Bose-Einstein condensates

A dark soliton becomes unstable when it is incident on a background density gradient, and the induced instability results in the emission of sound. Detailed quantitative studies of sound emission are performed for various potentials, such as steps, linear ramps and gaussian traps. The amount of sound emission is found to be a significant fraction of the soliton energy for typical potentials. Continuous emission of sound is found to lead to an apparent deformation of the soliton profile. The power emitted by the soliton is shown to be parametrised by the square of the displacement of the centre of mass of the soliton from its density minimum, thus highlighting the significance of the inhomogeneity-induced soliton deformation.

cond-mat↗

arXiv2003

Soliton-sound interactions in quasi-one-dimensional Bose-Einstein condensates

Longitudinal confinement of dark solitons in quasi-one-dimensional Bose-Einstein condensates leads to sound emission and reabsorption. We perform quantitative studies of the dynamics of a soliton oscillating in a tight dimple trap, embedded in a weaker harmonic trap. The dimple depth provides a sensitive handle to control the soliton-sound interaction. In the limit of no reabsorption, the power radiated is found to be proportional to the soliton acceleration squared. An experiment is proposed to detect sound emission as a change in amplitude and frequency of soliton oscillations.

cond-mat↗

arXiv2002

Decay of superfluid turbulence via Kelvin wave radiation and vortex reconnections

The elementary processes involved in the decay of superfluid turbulence in the limit of low temperature are studied by numerical simulations of vortex ring collisions. We find that small vortex rings produced by reconnections eventually annihilate in a collision where all their energy is converted into Kelvin waves and sound. We show that sound emission due to Kelvin waves also leads to a loss of vortex line length but this dissipation mechanism alone is too small to account for the experimentally observed decay.

cond-mat↗

arXiv2002

Josephson spectroscopy of a dilute Bose-Einstein condensate in a double-well potential

The dynamics of a Bose-Einstein condensate in a double-well potential are analysed in terms of transitions between energy eigenstates. By solving the time-dependent and time-independent Gross-Pitaevskii equation in one dimension, we identify tunnelling resonances associated with level crossings, and determine the critical velocity that characterises the resonance. We test the validity of a non-linear two-state model, and show that for the experimentally interesting case, where the critical velocity is large, the influence of higher-lying states is important.

cond-mat↗