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J. Dalibard

Publications and source records attributed to J. Dalibard.

53 records · Page 3Linked to original sources
arXiv2002

Dynamics of a single vortex line in a Bose-Einstein condensate

We study experimentally the line of a single quantized vortex in a rotating prolate Bose-Einstein condensate confined by a harmonic potential. In agreement with predictions, we find that the vortex line is in most cases curved at the ends. We monitor the vortex line leaving the condensate. Its length is measured as a function of time and temperature. For a low temperature, the survival time can be as large as 10 seconds. The length of the line and its deviation from the center of the trap are related to the angular momentum per particle along the condensate axis.

cond-mat.stat-mech↗
arXiv2002

Loading of a cold atomic beam into a magnetic guide

We demonstrate experimentally the continuous and pulsed loading of a slow and cold atomic beam into a magnetic guide. The slow beam is produced using a vapor loaded laser trap, which ensures two-dimensional magneto-optical trapping, as well as cooling by a moving molasses along the third direction. It provides a continuous flux larger than $10^9$ atoms/s with an adjustable mean velocity ranging from 0.3 to 3 m/s, and with longitudinal and transverse temperatures smaller than $100 μ$K. Up to $3 10^8$ atoms/s are injected into the magnetic guide and subsequently guided over a distance of 40 cm.

cond-mat↗
arXiv2002

Critical rotation of a harmonically trapped Bose gas

We study experimentally and theoretically a cold trapped Bose gas under critical rotation, i.e. with a rotation frequency close to the frequency of the radial confinement. We identify two regimes: the regime of explosion where the cloud expands to infinity in one direction, and the regime where the condensate spirals out of the trap as a rigid body. The former is realized for a dilute cloud, and the latter for a Bose-Einstein condensate with the interparticle interaction exceeding a critical value. This constitutes a novel system in which repulsive interactions help in maintaining particles together.

cond-mat.mes-hall↗
arXiv2001

The transverse breathing mode of an elongated Bose-Einstein condensate

We study experimentally the transverse monopole mode of an elongated rubidium condensate. Due to the scaling invariance of the non-linear Schrödinger (Gross-Pitaevski) equation, the oscillation is monochromatic and sinusoidal at short times, even under strong excitation. For ultra-low temperatures, the quality factor $Q=ω_0/γ_0$ can exceed 2000, where $ω_0$ and $γ_0$ are the mode angular frequency and damping rate. This value is much larger than any previously reported for other eigenmodes of a condensate. We also present the temperature variation of $ω_0$ and $γ_0$.

cond-mat.stat-mech↗
arXiv2001

Gravitational Wave Induced Rotation of the Plane of Polarization of Pulsar Signals

We derive in this an expression for the rotation of plane of polarization, of an electromagnetic wave, induced by the field of a gravitational wave propagating along the same direction $\approx \f{Gμd^2Ω^4}{3ø}$, $ø$ and $Ω$ being their respective frequencies using the geometrical optics limit of the wave equation for fields. Estimating the effect for the case of pulses from binary pulsars, one finds it too small to be observable, there could be other sources like inspiralling binary or an asymmetric neutron star where the effect could be in the observable region.

astro-ph↗
arXiv2001

Formation of quantized vortices in a gaseous Bose-Einstein condensate

Using a focused laser beam we stir a Bose-Einstein condensate confined in a magnetic trap. When the stirring frequency lies near the transverse quadrupolar mode resonance we observe the nucleation of vortices. When several vortices are nucleated, they arrange themselves in regular Abrikosov arrays, and in the limit of large quantum number the lattice structure is shown to produce a quantum velocity field approaching that for classical, rigid body rotation. Using a percussive excitation of the condensate, we measure the angular momentum of the condensate with vortices present and study the nucleation band as a function of the stirring intensity and geometry. We find that with only quadratic terms in the rotating perturbation the nucleation band is located around the quadrupolar resonance and has a width that increases with the strength of the stirring perturbation. However, when the potential includes cubic terms, the nucleation band broadens to include the hexapolar resonance as well. The results presented here demonstrate that the nucleation of vortices in the case of a harmonically trapped BEC is driven by the resonant excitation of the rotating quadrupolar mode, or by higher order rotating surface modes when the rotating perturbation includes the corresponding terms.

cond-mat.stat-mech↗
arXiv2001

An Atom Faucet

We have constructed and modeled a simple and efficient source of slow atoms. From a background vapour loaded magneto-optical trap, a thin laser beam extracts a continuous jet of cold rubidium atoms. In this setup, the extraction column that is typical to leaking MOT systems is created without any optical parts placed inside the vacuum chamber. For detailed analysis, we present a simple 3D numerical simulation of the atomic motion in the presence of multiple saturating laser fields combined with an inhomogeneous magnetic field. At a pressure of $P_{\rm Rb87}=1 \times 10^{-8}$ mbar, the moderate laser power of 10 mW per beam generates a jet of flux $Φ=1.3\times 10^8$ atoms/s with a mean velocity of 14 m/s and a divergence of $<20$ mrad.

physics.atom-ph↗
arXiv2001

Stationary states of a rotating Bose-Einstein condensate: routes to vortex nucleation

Using a focused laser beam we stir a $^{87}$Rb Bose-Einstein condensate confined in a magnetic trap. We observe that the steady states of the condensate correspond to an elliptic cloud, stationary in the rotating frame. These steady states depend nonlinearly on the stirring parameters (amplitude and frequency), and various solutions can be reached experimentally depending on the path followed in this parameter space. These states can be dynamically unstable and we observe that such instabilities lead to vortex nucleation in the condensate.

cond-mat.stat-mech↗
arXiv2000

Measurement of the angular momentum of a rotating Bose-Einstein condensate

We study the quadrupole oscillation of a Bose-Einstein condensate of $^{87}$Rb atoms confined in an axisymmetric magnetic trap, after it has been stirred by an auxiliary laser beam. The stirring may lead to the nucleation of one or more vortices, whose presence is revealed unambiguously by the precession of the axes of the quadrupolar mode. For a stirring frequency $Ω$ below the single vortex nucleation threshold $Ω_c$, no measurable precession occurs. Just above $Ω_c$, the angular momentum deduced from the precession is $\sim \hbar$. For stirring frequencies above $Ω_c$ the angular momentum is a smooth and increasing function of $Ω$, until an angular frequency $Ω_c'$ is reached at which the vortex lattice disappears and the precession stops.

cond-mat.soft↗
arXiv2000

Vortex lattices in a stirred Bose-Einstein condensate

We stir with a focused laser beam a Bose-Einstein condensate of $^{87}$Rb atoms confined in a magnetic trap. We observe the formation of a single vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to eleven vortices are simultaneously present. We present measurements of the decay of a vortex array once the stirring laser beam is removed.

cond-mat.stat-mech↗
arXiv2000

The N boson time dependent problem: an exact approach with stochastic wave functions

We present a numerically tractable method to solve exactly the evolution of a N boson system with binary interactions. The density operator of the system rho is obtained as the stochastic average of particular operators |Psi_1> are either Fock states |N:phi_{1,2}> or coherent states |coh:phi_{1,2}> with each particle in the state phi_{1,2}. We determine the conditions on the evolution of phi_{1,2} -which involves a stochastic element- under which we recover the exact evolution of rho. We discuss various possible implementations of these conditions. The well known positive P-representation arises as a particular case of the coherent state ansatz. We treat numerically two examples: a two-mode system and a one-dimensional harmonically confined gas. These examples, together with an analytical estimate of the noise, show that the Fock state ansatz is the most promising one in terms of precision and stability of the numerical solution.

cond-mat.soft↗
arXiv1999

Vortex formation in a stirred Bose-Einstein condensate

Using a focused laser beam we stir a Bose-Einstein condensate of 87Rb confined in a magnetic trap and observe the formation of a vortex for a stirring frequency exceeding a critical value. At larger rotation frequencies we produce states of the condensate for which up to four vortices are simultaneously present. We have also measured the lifetime of the single vortex state after turning off the stirring laser beam.

cond-mat.stat-mech↗
arXiv1999

Evaporative cooling of an atomic beam

We present a theoretical analysis of the evaporative cooling of an atomic beam propagating in a magnetic guide. Cooling is provided by transverse evaporation. The atomic dynamics inside the guide is analyzed by solving the Boltzmann equation with two different approaches: an approximate analytical ansatz and a Monte-Carlo simulation. Within their domain of validity, these two methods are found to be in very good agreement with each other. They allow us to determine how the phase-space density and the flux of the beam vary along its direction of propagation. We find a significant increase for the phase-space density along the guide for realistic experimental parameters. By extrapolation, we estimate the length of the beam needed to reach quantum degeneracy.

cond-mat.stat-mech↗
arXiv1999

Collective oscillations of a classical gas confined in harmonic traps

Starting from the Boltzmann equation we calculate the frequency and the damping of the monopole and quadrupole oscillations of a classical gas confined in an harmonic potential. The collisional term is treated in the relaxation time approximation and a gaussian ansatz is used for its evaluation. Our approach provides an explicit description of the transition between the hydrodynamic and collisionless regimes in both spherical and deformed traps. The predictions are compared with the results of a numerical simulation.

cond-mat↗
arXiv1998

Three-body decay of a rubidium Bose-Einstein condensate

We have measured the three-body decay of a Bose-Einstein condensate of rubidium ($^{87}$Rb) atoms prepared in the doubly polarized ground state $F=m_F=2$. Our data are taken for a peak atomic density in the condensate varying between $2\times 10^{14}$ cm$^{-3}$ at initial time and $7\times 10^{13}$ cm$^{-3}$, 16 seconds later. Taking into account the influence of the uncondensed atoms onto the decay of the condensate, we deduce a rate constant for condensed atoms $L=1.8 (\pm 0.5) \times 10^{-29}$ cm$^{6} $s$^{-1}$. For these densities we did not find a significant contribution of two-body processes such as spin dipole relaxation.

cond-mat.stat-mech↗
arXiv1998

Dynamics of two interacting Bose-Einstein condensates

We analize the dynamics of two trapped interacting Bose-Einstein condensates and indentify two regimes for the evolution: the regime of slow periodic oscillations and the regime of strong non-linear mixing leading to the damping of the relative motion of the condensates. We compare our predictions with an experiment recently performed at JILA.

cond-mat.stat-mech↗