> Magnetic quantum gases  Strontium
Magnetic quantum gases  Chromium
Bruno LaburtheTolra, Laurent Vernac, Etienne Maréchal, Robert Martin de Saint Vincent, Paolo Pedri, Olivier Gorceix, Youssef Aziz Alaoui (PhD), Lucas Gabardos (PostDoc), Kaci Kechadi (PhD)
Overview
Chromium atoms in their ground state have a large spin, and a large magnetic moment. Hence they have remarkable collisional properties, with large Van der Waals spin dependent interactions, and large dipoledipole interactions, compared to alkaline. Ultracold chromium gases have therefore specific properties that we investigate in experiments.
Recent results
Out of equilibrium spin dynamics
When atoms are prepared in an excited spin state, spin populations can change as a function of time. We have characterized the dynamics after rotation of all spins by an angle ? with respect to the magnetic field, in different configurations:
* Spin dynamics in a Bose Einstein Condensate (BEC)
In a chromium BEC, our experimental results demonstrate that a mean field approach describes accurately the dynamics. In particular, we observed a subtle effect of the dipolar interaction: spin dynamics is triggered by these interactions unless the rotation angle téta is equal to pi / 2. In presence of magnetic inhomogeneities, we observed a surprising behavior: spin dependent interactions preserve the initial ferromagnetic character of the BEC [1]. We then predicted and observed spin modes in this ferrofluid, with an oscillation frequency corresponding to the confinement energy in the trap. These collective modes can be seen as trapped magnons, where orbital and spin degrees of freedom are coupled, with spins precessing around their initial direction [2].
Caption: in presence of magnetic inhomogeneities, spins in a BEC rotate around their initial direction, with a position dependent amplitude (a). This collective behavior indicates a spin mode, where spin components populations (c) and spatial separation (d) oscillate with the same frequency.
* Spin dynamics in an optical lattice
By loading the BEC in a deep optical lattice, we obtain a Mott insulator state containing one atom per lattice site, in which each spin is coupled to many through the longrange dipolar interaction. In this manybody system, the spin exchange dynamics which develops cannot be reproduced by mean field simulations. On the other hand, quantum simulations show agreement with data, which demonstrate build up of quantum correlations, and entanglement. As entanglement grows in our spin system, entropy increases, and a stationary state is reached; this state has spin populations similar to a thermal state. All these features are compatible with the quantum thermalization process, in which entanglement leads to equilibrium of an isolated system [3].
We studied as well how spin dynamics is affected by transport, by reducing the lattice depth. Our data allow to observe a transition between the insulating regime, and the superfluid regime for which the Gutzwiller model is showed to qualitatively capture the rich physics at play [4].
Laser cooling
We implemented the grey molasses cooling technique, in which atoms lose energy as they climb potential hills resulting from laser polarization gradients. We obtained a record low temperature and high phase space density by cooling all degrees of freedom inside a trap [5].
Contacts
Bruno LaburtheTolra ou Laurent Vernac
References

S. Lepoutre, K. Kechadi, B. Naylor, B. Zhu, L. Gabardos, L. Isaev, P. Pedri, A. M. Rey, L. Vernac, and B. LaburtheTolra
Spin mixing and protection of ferromagnetism in a spinor dipolar condensate
Phys. Rev. A 97, 023610 (2018)

S. Lepoutre, L. Gabardos, K. Kechadi, P. Pedri, O. Gorceix, E. Maréchal, L. Vernac, and B. LaburtheTolra
Collective Spin Modes of a Trapped Quantum Ferrofluid
Phys. Rev. Lett. 121, 013201 (2018)

S. Lepoutre, J. Schachenmayer, L. Gabardos, B. Zhu, B. Naylor, E. Maréchal, O. Gorceix, A. M. Rey, L. Vernac and B. LaburtheTolra
Outofequilibrium quantum magnetism and thermalization in a spin3 manybody dipolar lattice system
Nature Communications 10, 1714 (2019)

P. Fersterer, A. SafaviNaini, B. Zhu, L. Gabardos, S. Lepoutre, L. Vernac, B. LaburtheTolra, P. Blair Blakie, and Ana Maria Rey
Dynamics of an itinerant spin3 atomic dipolar gas in an optical lattice
Phys. Rev. A 100, 033609 (2019)

L. Gabardos, S. Lepoutre, O. Gorceix, L. Vernac, and B. LaburtheTolra
Cooling all external degrees of freedom of optically trapped chromium atoms using gray molasses
Phys. Rev. A 99, 023607 (2019)
