Magnetic Atoms Quantum Simulators / QUANTERA CONSORTIUM MAQS

Members of the MAQS consortium include:

  * *Coordinator: Bruno Laburthe-Tolra (CNRS, FR)*
  * Tommaso Roscilde (ENS of Lyon, FR)
  * Francesca Ferlaino (Institut für Quantenoptik und Quanten-information, AT)
  * Tilman Pfau (Universität Stuttgart, DE)
  * Giovanni Modugno (Istituto Nazionale di Ottica, IT)
  * Maciej Lewenstein (Institute of Photonic Sciences, ES)
  * Mariusz Gajda (Instytut Fizyki Polskiej Akademii Nauk, PL)

We propose 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. We propose to develop a number of new tools to increase the strength of dipole-dipole interactions (shorter-period UV lattices, magneto-association of magnetic atoms into molecules with a stronger magnetic moment), and to control and measure their interaction at the nano-scale (using super-resolution techniques and narrow spectroscopic lines). Most importantly, we will develop new probes to certify the presence of quantum correlations, which are expected to be particularly strong in these many-body long-range interacting systems. Experimentally, we will either probe correlations in real space (microscope, double-well lattices), in momentum space (Doppler spectroscopy, time-of-flight), or in the spin sector. These probes will be developed in close collaboration with theory, to find the best ways to define and quantify entanglement.

  Working towards these aims, our results so far include: (i) the construction of two new quantum gas microscope experiments to probe Er and Dy individually, or in a mixture combination. (ii) the experimental characterization of correlations by measuring collective spin fluctuations. (iii) a number of new proposals to characterize entanglement in large spin systems, such as: methods relevant for quantum gas microscopes; data-driven approaches to reconstruct optimal Bell inequalities and entanglement criteria tailored on the input of experiments, based on collective measurements; methods to retrieve higher-order correlations from single-shot images; methods to reveal entanglement in momentum space. A number of new numerical methods have been devised (time-dependent variational approach, time-dependent Schwinger-boson approach) or implemented (DMRG, Exact Diagonalization, BCS mean-field), which allowed to explore out-of-equilibrium dynamics, and a variety of models with long-range interactions such as the extended Bose-Hubbard model, long-range Kitaev chains, long-range XXZ model, or phonon modes in polarized magnetic atoms localized in an optical lattice.

  These first achievements set us in a good way to complete our program, which is to show that lattice-trapped magnetic atoms can be used as quantum simulators, in order to investigate various families of problems. First, we our aim is to probe low energy phases, and second, out-of-equilibrium situations to investigate dynamics and quantum thermalization. Thanks to these improvements, a number of phases could now be within experimental reach, such as the supersolid or stripe phases, or peculiar phases of spin systems with long-range interactions. We will aim at protocols to certify the nature of the quantum correlations within these systems. Such correlations can be explored in four different complementary setups: 1) an Er lattice gas within a Dy bath (Innsbruck); strongly dipolar lattice gases made of either 2) Dy atoms in UV lattices (Stuttgart) or 3) Dy2 molecules in standard lattices (Pisa/Florence), and 4) Cr atoms realizing lattice spin models (Paris).

Overview of the chromium project activities 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.