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 : Martin RobertdeSaintVincent (group leader, CNRS)
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 (University Sorbonne Paris Nord), Bruno LaburtheTolra (CNRS)
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 RobertdeSaintVincent, Bruno LaburtheTolra (CNRS)
Under controlled conditions, the collective emission of light by an ensemble of emitters leads to intriguing effects, such as the spontaneous emergence of entanglement and altered emission properties. We will make use of the collective phenomenon called superradiance to create a source of light with ultrastable frequency and a linewidth even below that of individual emitters.Using a beam of strontium atoms passing through an optical cavity, we aim at bringing this "superradiant laser" to the continuous regime, where it may be used as a novel type of atomic clock. See more on the project here.
Contacts : Martin RobertdeSaintVincent, Benjamin Pasquiou, Bruno LaburtheTolra (CNRS)
THE THEORETICAL LAB:
Contact : Paolo Pedri (University Sorbonne Paris Nord )
Fundings:
Our activities are funded by the European Research Commission (QuantERA ERANET Cofund 2019), Agence Nationale de la Recherche (défi Technologies Quantiques 2019, TremplinERC 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 FIRSTTF.
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 (LNESyrte), J. Schachenmayer (Université de Strasbourg), T. Roscilde (Ecole Normale Supérieure de Lyon), A. Crubelier (Laboratoire Aimé Cotton).
Latest results (see complete list here):
Measuring correlations from the collective spin fluctuations of a large ensemble of latticetrapped dipolar spin3 atomsWe perform collective spin
measurements to study the buildup of twobody
correlations between Phys. Rev. Lett. 129, 023401 (2022) 

Measuring densities of cold atomic clouds smaller than the resolution limitWe propose and demonstrate an experimental method to measure by absorption imaging the size and local column density of a cloud of atoms, even when its smallest dimension is smaller than the resolution of the imaging system. To do this, we take advantage of the fact that, for a given total number of atoms, a smaller and denser cloud scatters less photons when the gas is optically thick. The method relies on making an ansatz on the cloud shape along the unresolved dimension(s), and on providing an additional information such as the total number of atoms. We demonstrate the method on insitu absorption images of quasi1D 87Sr Fermi gases. We find significant nonlinear corrections to the estimated size and local density of the cloud compared to a standard analysis. This allows us to recover an undistorted longitudinal density profile, and to measure transverse sizes as small as one fourth of our imaging resolution. The ultimate limit of our method is the wavelength that is used for imaging. 

Relaxation of the Collective Magnetization of a Dense 3D Array of Interacting Dipolar S=3 AtomsWe report on measurements of the dynamics of the total magnetization and spin populations in an almostunitfilled lattice system comprising about 10^{4} spin 3 chromium atoms, under the effect of dipolar interactions. The observed spin population dynamics is unaffected by the use of a spin echo and fullyconsistent with numerical simulations of the XXZ spin model. On the contrary, the observed magnetization 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 manybody behavior that cannot be diagnosed by merely measuring spin populations. 

Threepulse spin and momentum resolved picture of a fivecomponent Sr gas 
Adiabatic spindependent momentum transfer in an SU(N) degenerate Fermi gasWe introduce a spinorbit coupling scheme, where a retroreflected laser beam selectively diffracts two spin components in opposite directions. Spin sensitivity is provided by sweeping through a magneticfield sensitive transition while dark states ensure that spontaneous emission remains low. The scheme is adiabatic and thus inherently robust. This tailored spinorbit coupling allows simultaneous measurements of the spin and momentum distributions of a strontium degenerate Fermi gas, and thus opens the path to momentumresolved spin correlation measurements on SU(N) quantum magnets. 
Shelving spectroscopy of the strontium intercombination lineWe
present a spectroscopy scheme for the 7kHzwide
689nm 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 

Dynamics of an itinerant spin3 atomic dipolar gas in an optical latticeArrays of ultracold dipolar gases loaded in optical lattices are emerging as powerful quantum simulators of the manybody physics associated with the rich interplay between longrange dipolar interactions, contact interactions, motion, and quantum statistics. In this work we report on our investigation of the quantum manybody dynamics of a large ensemble of bosonic magnetic chromium atoms with spin S = 3 in a threedimensional 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 meanfield calculations based on the Gutzwiller ansatz; on the contrary, only a beyond meanfield theory can account for the dynamics at large lattice depths. While the crossover 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. 

Cooling All External Degrees of Freedom of Optically Trapped Chromium Atoms Using Gray MolassesWe 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 phasespace 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. 

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. Nature Comm. 10, 1714 (2019) 

Dissipative cooling of spin chains by a bath of dipolar particlesWe consider a spin chain of fermionic atoms in an optical lattice, interacting with each other by superexchange interactions. We theoretically investigate the dissipative evolution of the spin chain when it is coupled by magnetic dipoledipole 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. 

Collective spin modes of a trapped quantum ferrofluidWe report on the observation of a collective spin mode in a spinor BoseEinstein 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 zeropoint 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 spindependent contact interactions. Phys. Rev. Lett. 121, 013201 (2018) 
