The chromium BEC lab

Overview of the chromium BEC project activities

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 (chromium BEC) 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. 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.

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.

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. 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.

Production of a new dipolar Fermi 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 favorable interspecies scattering length to optimize evaporation of a Bose Fermi mixture. Loading of dipolar fermions in optical lattices offer us new possibilities for quantum magnetism studies.

Magnetic Atoms Quantum Simulators / QUANTERA CONSORTIUM MAQS

We are coordinator of a quantera project that proposes a quantum simulator made of magnetic atoms in periodic potentials, which will enable the investigation of quantum-many body problems associated with long-range dipole-dipole interactions.

You can learn more about this project here

Introduction to the chromium BEC 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 quantum magnetism. Furthermore, those systems are promising components for the quantum treatment of information. Ultracold atom physics is growing as a fascinating interdisciplinary domain.

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 studies, 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.

Project recent news

Probing coherences and itinerant magnetism
21/01/2025
We use dynamical decoupling to preserve spin coherence against magnetic fluctuations, which allows us to study itinerant magnetism of lattice-trapped magnetic atoms, driven by magnetic dipole-dipole interactions, in the low-entropy and close-to-unit filling regime. The results are presented in our new paper.

Project publications

Measuring correlations from the collective spin fluctuations of a large ensemble of lattice-trapped dipolar spin-3 atoms
06/07/2022
Ana Maria Rey, Bihui Zhu, Bruno Laburthe-Tolra, Laurent Vernac, Sean Robert Muleady, Tommaso Roscilde, William Dubosclard, Youssef Aziz Alaoui
We perform collective spin measurements to study the buildup of two-body correlations between ≈10^4 spin s=3 chromium atoms pinned in a 3D optical lattice. arXiv:2201.07277 (2022) / Phys. Rev. Lett. 129, 023401 (2022)
Dynamics of an itinerant spin-3 atomic dipolar gas in an optical lattice
14/05/2019
Ana Maria Rey, Arghavan Safavi-Naini, Bihui Zhu, Bruno Laburthe-Tolra, Laurent Vernac, Lucas Gabardos, P. Blair Blakie, Petra Fersterer, Steven Lepoutre
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. arXiv:1905.06123 (2019) / Phys. Rev. A 100, 033609 (2019)
Cooling all external degrees of freedom of optically trapped chromium atoms using gray molasses
21/12/2018
Bruno Laburthe-Tolra, Laurent Vernac, Lucas Gabardos, Olivier Gorceix, Steven Lepoutre
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. arXiv:1812.09177 (2018) / Phys. Rev. A 99, 023607 (2019)
Exploring out-of-equilibrium quantum magnetism and thermalization in a spin-3 many-body dipolar lattice system
07/03/2018
Ana Maria Rey, Bihui Zhu, Bruno Laburthe-Tolra, Bruno Naylor, Etienne Maréchal, Johannes Schachenmayer, Laurent Vernac, Lucas Gabardos, Olivier Gorceix, Steven Lepoutre
We study the spin dynamics and approach towards local thermal equilibrium of a macroscopic ensemble of S = 3 chromium atoms pinned in a three dimensional optical lattice and prepared in a pure coherent spin state, under the effect of magnetic dipole–dipole interactions. arXiv:1803.02628 (2018) / Nat. Commun. 10, 1714 (2019)
Probing spin dynamics from the Mott insulating to the superfluid regime in a dipolar lattice gas
15/07/2015
Arijit Sharma, Aurélie de Paz, Bruno Laburthe-Tolra, Bruno Naylor, Etienne Maréchal, Laurent Vernac, Maxim Efremov, Olivier Gorceix, Paolo Pedri
We analyze the spin dynamics of an out-of-equilibrium large spin dipolar atomic Bose gas in an optical lattice. We observe a smooth crossover from a complex oscillatory behavior to an exponential behavior throughout the Mott-to-superfluid transition. arXiv:1507.04273 (2015) / Phys. Rev. A 93, 021603(R) (2016)
Control of dipolar relaxation in external fields
01/02/2010
Anne Crubellier, Benjamin Pasquiou, Bruno Laburthe-Tolra, Etienne Maréchal, Gabriel Bismut, Laurent Vernac, Olivier Gorceix, Paolo Pedri, Quentin Beaufils
We study dipolar relaxation in both ultracold thermal and Bose-condensed Cr atom gases. We show three different ways to control dipolar relaxation, making use of either a static magnetic field, an oscillatory magnetic field, or an optical lattice to reduce the dimensionality of the gas from three-dimensional (3D) to two-dimensional (2D). arXiv:1002.0222 (2010) / Phys. Rev. A 81, 042716 (2010)
Radio-frequency-induced ground-state degeneracy in a Bose-Einstein condensate of chromium atoms
28/08/2008
Bruno Laburthe-Tolra, Etienne Maréchal, Jean-Claude Keller, Laurent Vernac, Olivier Gorceix, Quentin Beaufils, Radu Chicireanu, Thomas Zanon
We study the effect of strong radio-frequency (RF) fields on a Bose-Einstein condensate (BEC) of chromium atoms, in a regime where the RF frequency is much larger than the Larmor frequency. arXiv:0808.3931 (2008) / Phys. Rev. A 78, 051603(R) (200)
Accumulation and thermalization of cold atoms in a finite-depth magnetic trap
13/12/2006
Arnaud Pouderous, Bruno Laburthe-Tolra, Etienne Maréchal, J. V. Porto, Jean-Claude Keller, Laurent Vernac, Olivier Gorceix, Quentin Beaufils, Radu Chicireanu
We study the continuous accumulation of cold atoms from a magneto-optical trap (MOT) into a finite depth trap, consisting in a magnetic quadrupole trap dressed by a radiofrequency (RF) field. arXiv:0612133 (2006) / Phys. Rev. A 76, 023406 (2007)