The chromium project :

Results since 2013
Chromium atoms in their ground state have a large spin, and a large permanent magnetic dipole moment. The long range and anisotropic dipole-dipole interactions between the atoms confer to ultracold chromium gases unique properties.

An experimental platform for quantum systems simulation

By loading a chromium BEC in optical lattices, we have obtained a Mott insulator state comprising a dipolar species, and for the first time demonstrated intersite interactions between the atoms [1]. The dipolar spin exchange dynamics which takes place in this intrinsically many-body system is in agreement with our plaquette simulations taking into account quantum correlations. Our spin system is an excellent tool for quantum simulation, with an interplay between long-range dipolar and short range Van der Waals interactions. We varied the lattice depth from the superfluid to the Mott insulator regime to investigate the coupling between spin dynamics and transport [2].

Our recent research on this topic includes the study of the relaxation of spins after they are tilted with respect to their initial direction. The spins interact under the effect of dipole-dipole interactions, and the many-body system is thus an isolated system which relaxes due to inner forces. We have explored this scenario of quantum thermalization, where the final steady state corresponds to a thermal-like state whose apparent entropy is due to many-body entanglement. Our experiment is well captured by semiclassical simulations based on a discrete Monte Carlo sampling in phase space, that reveal the growth of entanglement during the thermalization process [3].

Control and use of the spin degrees of freedom
In a chromium BEC, inelastic dipolar collisions provide spin-orbit coupling which allows thermalizing the spin degrees of freedom. Thanks to this thermalization, we have demonstrated a new cooling mechanism, based on a purification of the BEC after transfer of thermal atoms in excited Zeeman states [4]. We also have investigated the interplay between spin dynamics and Bose condensation to create a multicomponent BEC when a fast shock cooling process is performed on a depolarized sample [5].

Production of a new dipolar Femi Sea
We have obtained the first chromium Fermi Sea with the 53Cr isotope, despite low isotopic abundance, and extreme complexity of the atomic structure due to hyperfine splitting. We have taken advantage of a favourable interspecies scattering length to optimize evaporation of a Bose Fermi mixture [6]. Loading of dipolar fermions in optical lattices offer us new possibilities for quantum magnetism studies.

Selection of publications: (see complete list here and abstracts here)

  1. De Paz A., Sharma A., Chotia A., Maréchal E., Huckans J.H., Pedri P., Santos L., Gorceix O., Vernac L., Laburthe-Tolra B.,
    Non-equilibrium quantum magnetism in a dipolar lattice gas,
    Physical review letters,  111 , 185305, (2013)
  2. de Paz A., Pedri P., Sharma A., Efremov M., Naylor B., Gorceix O., Maréchal E., Vernac L., Laburthe-Tolra B.,
    Probing spin dynamics from the Mott insulating to the superfluid regime in a dipolar lattice gas,
    Physical Review A Rapid Communications,  93 , 021603(R), (2016)

  3. S. Lepoutre, J. Schachenmayer, L. Gabardos, B. Zhu, B. Naylor, E. Maréchal, O. Gorceix, A. M. Rey, L. Vernac & B. Laburthe-Tolra,
    Out-of-equilibrium quantum magnetism and thermalization in a spin-3 many-body dipolar lattice system,
    Nature Communications volume 10, 1714 (2019) 

  4. Naylor B., Maréchal E., Huckans J.H., Gorceix O., Pedri P., Vernac L., Laburthe-Tolra B.,
    Cooling of a Bose-Einstein Condensate by spin distillation,
    Physical review letters,  115 , 243002, (2015)

  5. Naylor B., Brewczyk M., Gajda M., Gorceix O., Maréchal E., Vernac L., Laburthe-Tolra B.,
    Competition between Bose Einstein Condensation and spin dynamics,
    Physical review letters,  117 , 185302, (2016)

  6. Naylor B., Reigue A., Maréchal E., Gorceix O., Laburthe-Tolra B., Vernac L.,
    A chromium dipolar Fermi sea,
    Physical Review A,  91 , 011603(R), (2015)

Introduction to the chromium project:

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 also PICTURES of our experimental set-up.