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J. M. Vogels

Publications and source records attributed to J. M. Vogels.

18 recordsLinked to original sources
arXiv2009

Phase contrast imaging of Bose condensed clouds

Phase contrast imaging is used to observe Bose-Einstein condensates (BECs) at finite temperature in situ. The imaging technique is used to accurately derive the absolute phase shift of a probe laser beam due to both the condensate and the thermal cloud. The accuracy of the method is enhanced by using the periodicity of the intensity signal as a function of the accumulated phase. The measured density profiles can be described using a two relevant parameter fit, in which only the chemical potential and the temperature are to be determined. This allows us to directly compare the measured density profiles to different mean-field models in which the interaction between the condensed and thermal atoms is taken into account to various degrees.

cond-mat.quant-gas↗
arXiv2009

Second sound dipole mode in a partially Bose-Einstein condensed gas

We study the second sound dipole mode in a partially Bose-Einstein condensed gas. This mode is excited by spatially separating and releasing the center-of-mass of the Bose-Einstein condensate (BEC) with respect to the thermal cloud, after which the equilibration is observed. The oscillation frequency and the damping rate of this mode is studied for different harmonic confinements and temperatures. The measured damping rates close to the collisionless regime are found to be in good agreement with Landau damping. For increasing hydrodynamicity of the cloud we observe an increase of the damping.

cond-mat.quant-gas↗
arXiv2009

Enhanced heat flow in the hydrodynamic-collisionless regime

We study the heat conduction of a cold, thermal cloud in a highly asymmetric trap. The cloud is axially hydrodynamic, but due to the asymmetric trap radially collisionless. By locally heating the cloud we excite a thermal dipole mode and measure its oscillation frequency and damping rate. We find an unexpectedly large heat conduction compared to the homogeneous case. The enhanced heat conduction in this regime is partially caused by atoms with a high angular momentum spiraling in trajectories around the core of the cloud. Since atoms in these trajectories are almost collisionless they strongly contribute to the heat transfer. We observe a second, oscillating hydrodynamic mode, which we identify as a standing wave sound mode.

cond-mat.quant-gas↗
arXiv2009

Observation of shock waves in a large Bose-Einstein condensate

We observe the formation of shock waves in a Bose-Einstein condensate containing a large number of sodium atoms. The shock wave is initiated with a repulsive, blue-detuned light barrier, intersecting the BEC, after which two shock fronts appear. We observe breaking of these waves when the size of these waves approaches the healing length of the condensate. At this time, the wave front splits into two parts and clear fringes appear. The experiment is modeled using an effective 1D Gross-Pitaevskii-like equation and gives excellent quantitative agreement with the experiment, even though matter waves with wavelengths two orders of magnitude smaller than the healing length are present. In these experiments, no significant heating or particle loss is observed.

cond-mat.quant-gas↗
arXiv2007

Resonant superradiant backward-scattering as a source for many-particle entanglement

We investigate the atom pair production by superradiant backward-scattering from a Bose-Einstein condensate. By driving the superradiant process with two frequencies we can extend both the range of pulse duration and intensity by two orders of magnitude and obtain full control over the number of scattered atoms in forward and backward direction. We show that the atoms scattered in forward direction are strongly correlated with the atoms scattered in backward direction, which makes resonant superradiant backward-scattering a promising candidate for many-particle entanglement.

cond-mat.other↗
arXiv2006

Reaching the hydrodynamic regime in a Bose-Einstein condensate by suppression of avalanche

We report the realization of a Bose-Einstein condensate (BEC) in the hydrodynamic regime. The hydrodynamic regime is reached by evaporative cooling at a relative low density suppressing the effect of avalanches. With the suppression of avalanches a BEC containing 120.10^6 atoms is produced. The collisional opacity can be tuned from the collisionless regime to a collisional opacity of more than 3 by compressing the trap after condensation. In the collisional opaque regime a significant heating of the cloud at time scales shorter than half of the radial trap period is measured. This is direct proof that the BEC is hydrodynamic.

cond-mat.other↗
arXiv2006

Large atom number Bose-Einstein condensate of sodium

We describe the setup to create a large Bose-Einstein condensate containing more than 120x10^6 atoms. In the experiment a thermal beam is slowed by a Zeeman slower and captured in a dark-spot magneto-optical trap (MOT). A typical dark-spot MOT in our experiments contains 2.0x10^10 atoms with a temperature of 320 microK and a density of about 1.0x10^11 atoms/cm^3. The sample is spin polarized in a high magnetic field, before the atoms are loaded in the magnetic trap. Spin polarizing in a high magnetic field results in an increase in the transfer efficiency by a factor of 2 compared to experiments without spin polarizing. In the magnetic trap the cloud is cooled to degeneracy in 50 s by evaporative cooling. To suppress the 3-body losses at the end of the evaporation the magnetic trap is decompressed in the axial direction.

physics.atom-ph↗
arXiv2006

Spin-Polarizing Cold Sodium Atoms in a Strong Magnetic Field

The efficiency of evaporative cooling, which is used for the creation of a Bose Einstein condensate, depends strongly on the number of particles at the start of the evaporation. A high efficiency can be reached by filling the magneto-optical trap with a large number of atoms and subsequently transferring these atoms to the magnetic trap as efficiently as possible. In our case (for sodium) this efficiency is limited to 1/3, because the magnetic substates of the F = 1 state, which is used in the trapping process, are equally populated. This limit can be overcome by spin-polarizing the sample before the transfer. For sodium atoms, however, the improvement is very small when it is done in a small magnetic field due to the large number of optical transitions in combination with the high optical density. In this article we describe spin-polarizing sodium atoms in a high magnetic field. The transfer efficiency is increased by a factor of 2. The high magnetic field makes the process also more robust against variations in the magnetic field, the laser frequency and the polarization of the laser beam.

physics.atom-ph↗
arXiv2004

Realization of a magnetically guided atomic beam in the collisional regime

We describe the realization of a magnetically guided beam of cold rubidium atoms, with a flux of $7\times 10^9$ atoms/s, a temperature of 400 $μ$K and a mean velocity of 1 m/s. The rate of elastic collisions within the beam is sufficient to ensure thermalization. We show that the evaporation induced by a radio-frequency wave leads to appreciable cooling and increase in phase space density. We discuss the perspectives to reach the quantum degenerate regime using evaporative cooling.

cond-mat.other↗
arXiv2004

How to reach the collisional regime on a magnetically guided atomic beam

In this paper, we report our progress towards the realization of a continuous guided atomic beam in the degenerate regime. So far, we have coupled into a magnetic guide a flux of a few $10^{8}$ atoms/s at 60 cm/s with a propagation in the guide over more than 2 meters. At this stage, the collision rate is not high enough to start an efficient forced evaporative cooling. Here we describe a new approach to reach the collisional regime. It is based on a pulsed feeding of the magnetic guide at a high repetition rate. The overlap of the packets of atoms occurs in the guide and leads to a continuous guided beam. We discuss different ways to increase the collision rate of this beam while keeping the phase space density constant by shaping the external potential.

cond-mat.other↗
arXiv2002

Amplification of Local Instabilities in a Bose-Einstein Condensate with Attractive Interactions

We study the collapse of large homogeneous Bose-Einstein condensates due to intrinsic attractive interactions. We observe the amplification of a local instability by seeding a momentum state $\bf{p}$ and suddenly switching the scattering length negative via a Feshbach resonance. As required by momentum conservation, we also observe the appearance of atoms in the conjugate momentum state. The time scale for this depletion process is found to be comparable to that for global collapse, implying that this process will be the primary decay channel for large homogeneous condensates.

cond-mat↗
arXiv2002

Coherent Collisions between Bose-Einstein Condensates

We study the non-degenerate parametric amplifier for matter waves, implemented by colliding two Bose-Einstein condensates. The coherence of the amplified waves is shown by observing high contrast interference with a reference wave and by reversing the amplification process. Since our experiments also place limits on all known sources of decoherence, we infer that relative number squeezing is most likely present between the amplified modes. Finally, we suggest that reversal of the amplification process may be used to detect relative number squeezing without requiring single-particle detection.

cond-mat.soft↗
arXiv2001

Vortex Nucleation in a Stirred Bose-Einstein Condensate

We studied the nucleation of vortices in a Bose-Einstein condensate stirred by a laser beam. We observed the vortex cores using time-of-flight absorption imaging. By varying the size of the stirrer, we observed either discrete resonances or a broad response as a function of the frequency of the stirrer's motion. Stirring beams small compared to the condensate size generated vortices below the critical rotation frequency for the nucleation of surface modes, suggesting a local mechanism of generation. In addition, we observed the centrifugal distortion of the condensate due to the rotating vortex lattice and found evidence for bent vortices.

cond-mat.soft↗
arXiv2001

Realization of Bose-Einstein condensates in lower dimensions

Bose-Einstein condensates of sodium atoms have been prepared in optical and magnetic traps in which the energy-level spacing in one or two dimensions exceeds the interaction energy between atoms, realizing condensates of lower dimensionality. The cross-over into two-dimensional and one-dimensional condensates was observed by a change in aspect ratio and saturation of the release energy when the number of trapped atoms was reduced.

cond-mat.soft↗
arXiv2000

Dissipationless flow and superfluidity in gaseous Bose-Einstein condensates

We study dissipation in a dilute Bose gas induced by the motion of a macroscopic object. A blue-detuned laser beam focused on the center of a trapped gas of sodium atoms was scanned both above and below the BEC transition temperature. The measurements allow for a comparison between the heating rates for the superfluid and normal gas.

cond-mat.soft↗
arXiv2000

Observation of Superfluid Flow in a Bose-Einstein Condensed Gas

We have studied the hydrodynamic flow in a Bose-Einstein condensate stirred by a macroscopic object, a blue detuned laser beam, using nondestructive {\em in situ} phase contrast imaging. A critical velocity for the onset of a pressure gradient has been observed, and shown to be density dependent. The technique has been compared to a calorimetric method used previously to measure the heating induced by the motion of the laser beam.

cond-mat.soft↗