Our research activities concentrate on experiments in the field of ultracold atom gases. We employ them as platforms for studying quantum manybody issues at the frontier of condensed matter physics. Current work focuses on quantum magnetism and we aim at producing strongly correlated states.
Contact : Bruno LaburtheTolra ( group leader, chargé de recherche CNRS)
The Chromium experiment is fully operating (see recent results below). In recent years, we performed various studies of dipolar effects in condensates made of spin3 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 BoseEinstein condensates. See more info on the project here.
Contact : Laurent Vernac ( Maitre de conférences )
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 THEORETICAL LAB:
Contact : Paolo Pedri ( Maitre de conférences )
Recent results:
Exploring outofequilibrium quantum magnetism and thermalization in a spin3 manybody dipolar lattice systemHere we experimentally study the dynamics and approach towards thermal equilibrium of a macroscopic ensemble of spins initially tilted compared to the magnetic field, under the effect of dipoledipole interactions. The experiment uses a unit filled array of 10^4 chromium atoms in a 3D optical lattice, realizing the spin3 XXZ Heisenberg model. We monitor the population of the seven spin components after a collective rotation of an initially polarized ensemble, as a function of the angle between the initial coherent state with respect to the magnetic field. We find that the approach to thermal equilibrium is increasingly driven by quantum correlations as the angle approaches pi/2.


Spin mixing and protection of ferromagnetism in a spinor dipolar condensateWe study spin mixing dynamics in a chromium dipolar BoseEinstein Condensate, after tilting the atomic spins by an angle θ with respect to the magnetic field. Spin mixing is triggered by dipolar coupling, but, once dynamics has started, it is mostly driven by contact interactions. For the particular case θ = π/ 2 , an external spinorbit coupling term induced by a magnetic gradient is required to enable the dynamics. Then the initial ferromagnetic character of the gas is locally preserved, an unexpected feature that we attribute to large spindependent contact interactions.


Cooling of a BoseEinstein Condensate by spin distillationWe propose and experimentally demonstrate a new cooling mechanism leading to purification of a spinor BoseEinstein 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 spinexcited 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.


Chromium dipolar Fermi seaWe 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.


Nonequilibrium quantum magnetism in a dipolar lattice gasResearch on quantum magnetism with ultracold gases in optical lattices is expected to open fascinating perspectives for the understanding of fundamental problems in condensedmatter physics. Here we report on the first realization of quantum magnetism using a degenerate dipolar gas in an optical lattice. In contrast to their nondipolar counterparts, dipolar lattice gases allow for intersite spinspin interactions without relying on superexchange 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 tJ model, which is characterized by a nonequilibrium spinor dynamics resulting from intersite Heisenberglike spinspin interactions provided by nonlocal dipoledipole interactions. Moreover, due to its large spin, chromium lattice gases constitute an excellent environment for the study of quantum magnetism of highspin systems, as illustrated by the complex spin dynamics observed for doublyoccupied sites.


Resonant demagnetization of a dipolar BEC in a 3D optical latticeWe 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 lineshape is sensitive to the onsite interaction energy. We use such sensitivity to probe number squeezing in a Mott insulator, and we reveal the production of threebody states with entangled spin and orbital degrees of freedom.


Anisotropic excitation spectrum of a dipolar quantum Bose gasWe measure the excitation spectrum of a dipolar Chromium Bose Einstein Condensate with RamanBragg spectroscopy. The energy spectrum depends on the orientation of the dipoles with respect to the excitation momentum, demonstrating an anisotropy which originates from the dipoledipole 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 GrossPitaevskii equation. Our results show an anisotropy of the speed of sound

Our team has constructed an experimental setup to generate BoseEinstein 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.
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 ^{53}Cr. We have already shown that our experimental setup allow to prepare at the same time a mixture of cold fermions and bosons.
(see recent results page)
see also PICTURES of our experimental setup.