Accueil > Groupes de recherche > Gaz quantiques > Expériences > Interaction and disorder
Disordered quantum gases with tunable interaction
Permanents: Thomas Bourdel (contact), Alain Aspect,
PhDs: Guillaume Berthet
We are especially looking for a M2/PhD student and can also offer a post-doc position.
We especially study gases in reduce dimensions (1D, 2D), where quantum and thermal fluctuations are most important. Importantly, we work with potassium 39, a bosonic element having nice magnetically tunable Feshbach resonances, thus permitting the control of the interatomic interaction. We can add a controllable amount of disorder thought a speckle light field. Disorder competes with the interaction induced superfluidity. For example a superfluid to insulator transition is observed in 2D Bose gases. We can also study non-interacting phenomena such as Anderson localization, i.e. the absence of diffusion due to multiple quantum interference. Recently, we have studied the superfluid propagation in a 1D disordered trap of bright solitons; condensed clouds of atoms which remain together because of attractive interaction (click to see a video).
In April 2017, we have changed the experimental setup, replacing the stainless steel science chamber with a small glass cell (click to see a video). The new setup permits the production of condensates every 6-7 seconds and it allows for high numerical aperture imaging, fast change of the magnetic field and easy RF transfers between spin states, thus allowing quenches of the interaction parameters.
Histograms of the reflected fraction of slow moving atoms in a disorder potential. a) atoms in a soliton. c) non-interacting atoms. | We observe nonlinear scattering of 39K atomic bright solitons launched in a one-dimensional (1D) speckle disorder. We directly compare it with the scattering of non-interacting particles in the same disorder. The atoms in the soliton tend to be collectively either reflected or transmitted, in contrast with the behavior of independent particles, thus demonstrating a clear nonlinear effect in scattering. The observed strong fluctuations in the reflected fraction, between zero and 100%, are interpreted as a consequence of the strong sensitivity of the system to the experimental conditions and in particular to the soliton velocity. This behavior is reproduced in a mean-field framework by Gross-Pitaevskii simulations. |
Propagation of 39K bright solitons. | We report on the production of 39K matter-wave bright solitons, i.e., 1D matter-waves that propagate without dispersion thanks to attractive interactions. Our solitons, close to the collapse threshold, are strongly bound and will find applications in fundamental physics and atom interferometry. |
Size of the condensate after expansion for a variable interaction parameter. | We have achieved Bose-Einstein condensation of potassium 39 in an all-optical cooling scheme. It permits to reduce the duration of the experimental cycles to a few seconds. The interactions between atoms are controlled in order to optimize the collision rate during the evaporation and can then be tuned to any desirable value. As an example, we have studied the radial expansion of the gas from an elongated trap exploring the 1D to 3D crossover. |
Temperature of the D1 gray molasses as a function of the D1 cooling intensity per beam. | We have developed new techniques in cooling 39K atoms using laser light close to the D1 transition. First, a new compressed-MOT configuration is taking advantage of gray molasses type cooling induced by blue-detuned D1 light. It yields an optimized density of atoms. Then, we use pure D1 gray molasses to further cool the atoms to an ultra-low temperature of 6 µK. The resulting phase-space density is 2x10-4 and will ease future experiments with ultracold potassium. As an example, we use it to directly load up to 3x107 atoms in a far detuned optical trap, a result that opens the way to the all-optical production of potassium degenerate gases. |
Typical example of a speckle disorder. The superfluid phase transition can be studied as a function of the disorder strength. | We experimentally study the effect of disorder on trapped quasi two-dimensional (2D) 87Rb clouds in the vicinity of the Berezinskii-Kosterlitz-Thouless (BKT) phase transition. The disorder correlation length is of the order of the Bose gas characteristic length scales (thermal de Broglie wavelength, healing length) and disorder thus modifies the physics at a microscopic level. We analyze the coherence properties of the cloud through measurements of the momentum distributions, for two disorder strengths, as a function of its degeneracy. For moderate disorder, the emergence of coherence remains steep but is shifted to a lower entropy. In contrast, for strong disorder, the growth of coherence is hindered. Our study is an experimental realization of the dirty boson problem in a well controlled atomic system suitable for quantitative analysis. |
2D Momentum distribution for different atom number. The left image corresponds to a normal gas, the middle one to a gas at the superfluid transition and the right one to a gas deep in the superfluid phase. The shrinking of the momentum distribution is related to the increasing coherence in the 2D gas when crossing the BKT superfluid transition. | We measure the momentum distribution of a 2D trapped Bose gas and observe the increase of the range of coherence around the Berezinskii-Kosterlitz-Thouless (BKT) transition. We quantitatively compare our observed profiles to both a Hartree-Fock mean-field theory and to quantum Monte-Carlo simulations. In the normal phase, the momentum distribution is observed to sharpen well before the phase transition. This behavior is partially captured in a mean-field approach, in contrast to the physics of the BKT transition. |
If you are interested in our research, please do not hesitate to contact Thomas Bourdel. We have open positions for PhD or post-docs. We also offer to some motivated undergraduate students at various levels to work with us every year. See the link to a more detail thesis/intership proposition above.
Group meetings take place every monday at 11:00 in Salle du conseil (2nd floor)
Institut d’Optique Graduate School, April 4th 2016.