STUDIES OF QUANTUM GASES

Our research activities concentrate on experiments in the field of ultra-cold atom gases. We employ them as platforms for studying quantum many-body issues at the frontier of condensed matter physics. Current work focuses on quantum magnetism and we aim at producing strongly correlated states.

Contact : Bruno Laburthe-Tolra ( group leader, chargé de recherche CNRS)

THE CHROMIUM LAB :

The Chromium experiment is fully operating (see results below) . In recent years, we performed various studies of dipolar effects in condensates made of spin-3 chromium atoms with a special interest into the properties of spinor quantum gases in bulk or in optical lattices. We take advantage of the remarkable properties of chromium atoms (high spin, large magnetic moment) to demonstrate new physical effects triggered by the interactions between dipoles inside the chromium Bose-Einstein condensates. We are now on the verge of extending recent work carried on with the bosonic isotope 52 Cr to the fermionic isotope 53 Cr that we recently brought down to degeneracy. The exploration of the rich magnetic phase diagram of this system is most promising.

Contact : Laurent Vernac ( Maitre de conférences )

THE STRONTIUM LAB :

On the strontium experiment, our objectives comprise the production of a degenerate Fermi gas of 87 Sr atoms, the reduction of the system entropy through innovative techniques, the use of the narrow lines of Sr to probe the system properties with an unprecedented precision (in collaboration with the Metrology group of our lab) and the creation of exotic magnetic materials. We are currently building the Strontium machine. See more on the project here.

Contact : Martin Robert de Saint Vincent ( Chargé de recherche CNRS)

THE THEORITICAL LAB:

Contact : Paolo Pedri ( Maitre de conférences )

 

Recent results:

Cooling of a Bose-Einstein Condensate by spin distillation

We propose and experimentally demonstrate a new cooling mechanism leading to purification of a spinor Bose-Einstein Condensate (BEC). Our scheme starts with a BEC polarized in the lowest energy spin state. Spin excited states are thermally populated by lowering the single particle energy gap set by the magnetic field. Then these spin-excited thermal components are filtered out, which leads to an increase of the BEC fraction. We experimentally demonstrate such cooling for a spin 3 chromium dipolar BEC. Our scheme should be applicable to Na or Rb, with perspective to reach temperatures below 1 nK.

Publication

 

Chromium dipolar Fermi sea

We report on the production of a degenerate Fermi gas of Chromium 53 atoms, polarized in the state F=9/2, mF=-9/2, by sympathetic cooling with bosonic S=3,mS=-3 Chromium 52 atoms. We load in an optical dipole trap 3×10^4 Chromium 53 atoms with 10^6 Chromium 52 atoms. Despite the initial small number of fermionic atoms, we reach a final temperature of TFinal=0.6×TF (Fermi temperature), with up to 10^3 Cr53 atoms. This surprisingly efficient evaporation stems from an interisotope scattering length |aBF|=80(±10)aB (Bohr radius) which is small enough to reduce evaporative losses of the fermionic isotope, but large enough to assure thermalization.

Publication

 

Non-equilibrium quantum magnetism in a dipolar lattice gas

Research on quantum magnetism with ultra-cold gases in optical lattices is expected to open fascinating perspectives for the understanding of fundamental problems in condensed-matter physics. Here we report on the first realization of quantum magnetism using a degenerate dipolar gas in an optical lattice. In contrast to their non-dipolar counterparts, dipolar lattice gases allow for inter-site spin-spin interactions without relying on super-exchange energies, which constitutes a great advantage for the study of spin lattice models. In this paper we show that a chromium gas in a 3D lattice realizes a lattice model resembling the celebrated t-J model, which is characterized by a non-equilibrium spinor dynamics resulting from inter-site Heisenberg-like spin-spin interactions provided by non-local dipole-dipole interactions. Moreover, due to its large spin, chromium lattice gases constitute an excellent environment for the study of quantum magnetism of high-spin systems, as illustrated by the complex spin dynamics observed for doubly-occupied sites.

Publication

 

Resonant demagnetization of a dipolar BEC in a 3D optical lattice

We study dipolar relaxation of a chromium BEC loaded into a 3D optical lattice. We observe dipolar relaxation resonances when the magnetic energy released during the inelastic collision matches an excitation towards higher energy bands. A spectroscopy of these resonances for two orientations of the magnetic field provides a 3D band spectroscopy of the lattice. The narrowest resonance is registered for the lowest excitation energy. Its line-shape is sensitive to the on-site interaction energy. We use such sensitivity to probe number squeezing in a Mott insulator, and we reveal the production of three-body states with entangled spin and orbital degrees of freedom.

Publication

 

Anisotropic excitation spectrum of a dipolar quantum Bose gas

We measure the excitation spectrum of a dipolar Chromium Bose Einstein Condensate with Raman-Bragg spectroscopy. The energy spectrum depends on the orientation of the dipoles with respect to the excitation momentum, demonstrating an anisotropy which originates from the dipole-dipole interactions between the atoms. We compare our results with the Bogoliubov theory based on the local density approximation, and, at large excitation wavelengths, with numerical simulations of the time dependent Gross-Pitaevskii equation. Our results show an anisotropy of the speed of sound

Publication

 

 

(see other results)

A dipolar condensate:

Our team has constructed an experimental setup to generate Bose-Einstein condensates (BECs) made of Chromium atoms. These atoms bear unusual properties due to their exceptionally high magnetic dipole moment.  By transferring the chromium BECs into optical lattices, we create and study artificial systems of perfect purity and valuable tunability. Indeed, we can change almost at will their temperature, density, interactions, confining potential strength and shape, etc. Such systems mimic complex systems at the heart of modern condensed matter physics, in particular those related to quatum magnetism. Furthermore, those systems are promising components for the quantum treatment of information. Ultracold atom physics is growing as a fascinating interdisciplinary domain.

Fig 1 : formation of the chromium BEC by forced evaporation in an optical trap.

Studies using a chromium BEC:

The chromium BEC allow us to performed different sudies, using the specificities of chromium. The field of quantum dipolar gases offers many opportunities for research that we are exploring with a particularly strong interest for the transfer of quantum dipolar gases into optical lattices (1D, 2D and 3D).

Another attractive issue is the realization of a Fermi sea with the fermionic isotope 53Cr. We have already shown that our experimental set-up allow to prepare at the same time a mixture of cold fermions and bosons.

(see recent results page)

see also PICTURES of our experimental set-up.