MAGNETIC 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, CNRS)

THE CHROMIUM LAB :      [OPEN POST-DOC POSITION]

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. See more info on the project here.

Contact : Laurent Vernac (University Sorbonne Paris Nord)

THE STRONTIUM LAB :      [OPEN PhD POSITION]

The strontium experiment is designed to study the quantum magnetism of large spin (9/2) fermionic particles, with an original SU(N) symmetry that reflects the invariance of interactions by any spin rotation. We reached Fermi degeneracy in spring 2019. We are now investigating the use of the narrow lines of strontium to manipulate and probe the spin degree of freedom of these atoms, exploiting the extraordinary degree of energy sensitivity afforded by metrology tools (in collaboration with the Metrology group of our lab). These tools will be applied in optical lattices for the preparation and study of novel magnetic materials.  See more on the project here.

Contact : Martin Robert de Saint Vincent (CNRS)

THE THEORETICAL LAB:

Contact : Paolo Pedri ( University Sorbonne Paris Nord )


Fundings:

Our activities are funded by the European Research Commission (QuantERA ERA-NET Cofund 2019), Agence Nationale de la Recherche (défi Technologies Quantiques 2019, Tremplin-ERC 2017), Region Île de France through the Institut FRancilien des Atomes Froids and the Domaines d'Intérêts Majeurs SIRTEQ and DIM Nano'K, IFCPAR, and Labex FIRST-TF.

Collaborations:

A. M. Rey (JILA, University of Colorado), M. Gajda (Institute of Physics, Polish Academy of Sciences), J. Huckans (University of Pennsylvania), P. B. Blakie (University of Otago), M. Cheneau (Laboratoire Charles Fabry), R. Le Targat, J. Lodewyck  (LNE-Syrte), J. Schachenmayer (Université de Strasbourg), T. Roscilde (Ecole Normale Supérieure de Lyon), A. Crubelier (Laboratoire Aimé Cotton).


Latest results (see complete list here):



Relaxation of the Collective Magnetization of a Dense 3D Array of Interacting Dipolar S=3 Atoms

We report on measurements of the dynamics of the total magnetization and spin populations in an almostunit-filled lattice system comprising about104spinS¼3chromium atoms, under the effect of dipolarinteractions. The observed spin population dynamics is unaffected by the use of a spin echo and fullyconsistent with numerical simulations of theS¼3XXZspin model. On the contrary, the observedmagnetization decays slower than in simulations and, surprisingly, reaches a small but nonzero asymptoticvalue within the longest timescale. Our findings show that spin coherences are sensitive probes tosystematic effects affecting quantum many-body behavior that cannot be diagnosed by merely measuringspin populations.

                                                        Phys. Rev. Lett. 125, 143401 (2020)


Three-pulse spin and momentum resolved picture
of a five-component Sr gas

Adiabatic spin-dependent momentum transfer in an SU(N) degenerate Fermi gas

We introduce a spin-orbit coupling scheme, where a retro-reflected laser beam selectively diffracts two spin components in opposite directions. Spin sensitivity is provided by sweeping through a magnetic-field sensitive transition while dark states ensure that spontaneous emission remains low. The scheme is adiabatic and thus inherently robust. This tailored spin-orbit coupling allows simultaneous measurements of the spin and momentum distributions of a strontium degenerate Fermi gas, and thus opens the path to momentum-resolved spin correlation measurements on SU(N) quantum magnets.

                                                        Phys. Rev. A 102, 013317 (2020)


Shelving spectroscopy of the strontium intercombination line

We present a spectroscopy scheme for the 7-kHz-wide 689-nm intercombination line of strontium. We rely on shelving detection, where electrons are first excited to a metastable state by the spectroscopy laser before their state is probed using the broad transition at 461 nm. As in the similar setting of calcium beam clocks, this enhances dramatically the signal strength as compared to direct saturated fluorescence or absorption spectroscopy of the narrow line. We implement shelving spectroscopy both in directed atomic beams and hot vapor cells with isotropic atomic velocities. We measure a fractional frequency instability 2×1012 at 1 s limited by technical noise - about one order of magnitude above shot noise limitations for our experimental parameters. Our work illustrates the robustness and flexibility of a scheme that can be very easily implemented in the reference cells or ovens of most existing strontium experiments, and may find applications for low-complexity clocks.

                                   J. Phys. B: At. Mol. Opt. Phys. 53 085005 (2020)


Dynamics of an itinerant spin-3 atomic dipolar gas in an optical lattice

Arrays of ultra-cold dipolar gases loaded in optical lattices are emerging as powerful quantum simulators of the many-body physics associated with the rich interplay between long-range dipolar interactions, contact interactions, motion, and quantum statistics. In this work we report on our investigation of the quantum many-body dynamics of a large ensemble of bosonic magnetic chromium atoms with spin S = 3 in a three-dimensional lattice as a function of lattice depth. Using extensive theory and experimental comparisons we study the dynamics of the population of the different Zeeman levels and the total magnetization of the gas across the superfluid to the Mott insulator transition. We are able to identify two distinct regimes: At low lattice depths, where atoms are in the superfluid regime, we observe that the spin dynamics is strongly determined by the competition between particle motion, on site interactions and external magnetic field gradients. Contact spin dependent interactions help to stabilize the collective spin length, which sets the total magnetization of the gas. On the contrary, at high lattice depths, transport is largely frozen out. In this regime, while the spin populations are mainly driven by long range dipolar interactions, magnetic field gradients also play a major role in the total spin demagnetization. We find that dynamics at low lattice depth is qualitatively reproduced by mean-field calculations based on the Gutzwiller ansatz; on the contrary, only a beyond mean-field theory can account for the dynamics at large lattice depths. While the cross-over between these two regimes does not correspond to sharp features in the observed dynamical evolution of the spin components, our simulations indicate that it would be better revealed by measurements of the collective spin length.

                                               Phys. Rev. A 100, 033609 (2019)


Cooling All External Degrees of Freedom of Optically Trapped Chromium Atoms Using Gray Molasses

We report on a scheme to cool and compress trapped clouds of highly magnetic 52Cr atoms. This scheme combines sequences of gray molasses, which freeze the velocity distribution, and free evolutions in the (close to) harmonic trap, which periodically exchange the spatial and velocity degrees of freedom. Taken together, the successive gray molasses pulses cool all external degrees of freedom, which leads to an increase of the phase-space density (PSD) by a factor of about 250, allowing to reach a high final PSD of about 1.7*10^-3. These experiments are performed within an optical dipole trap, in which gray molasses work equally well as in free space. The obtained samples are then an ideal starting point for the evaporation stage aiming at the quantum regime.

                                               Phys. Rev. A 99, 023607 (2019)

Exploring out-of-equilibrium quantum magnetism and thermalization in a spin-3 many-body dipolar lattice system

Here 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 dipole-dipole interactions. The experiment uses a unit filled array of 10^4 chromium atoms in a 3D optical lattice, realizing the spin-3 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.

Nature Comm. 10, 1714 (2019)

Dissipative cooling of spin chains by a bath of dipolar particles

We consider a spin chain of fermionic atoms in an optical lattice, interacting with each other by super-exchange interactions. We theoretically investigate the dissipative evolution of the spin chain when it is coupled by magnetic dipole-dipole interaction to a bath consisting of atoms with a strong magnetic moment. Dipolar interactions with the bath allow for a dynamical evolution of the collective spin of the spin chain. Starting from an uncorrelated thermal sample, we demonstrate that the dissipative cooling produces highly entangled low energy spin states of the chain in a timescale of a few seconds.

New Journal of Physics 20, 073037 (2018)

Collective spin modes of a trapped quantum ferrofluid

We report on the observation of a collective spin mode in a spinor Bose-Einstein condensate. Initially, all spins point perpendicular to the external magnetic field. The lowest energy mode consists in a sinusoidal oscillation of the local spin around its original axis, with an oscillation amplitude that linearly depends on the spatial coordinates. The frequency of the oscillation is set by the zero-point kinetic energy of the BEC. The observations are in excellent agreement with hydrodynamic equations. The observed spin mode has a universal character, independent of the atomic spin and spin-dependent contact interactions.

 Phys. Rev. Lett. 121, 013201 (2018)

 

(complete list here)