Séminaires

Title of seminar:  (Cold) Molecules as a tool to probe fundamental physics

Abstract:

Modern physics is struck with a paradox: there are very good reasons to believe that our current understanding of Nature is incomplete, yet it is exceedingly difficult to conduct an experiment which contradicts our best theories. This tension is particularly palpable since the Large Hadron Collider explored a wide range of energies without a clear detection of new physics, thereby discarding many extensions Beyond the Standard Model (BSM). Searching for new physics in the high energy domain now demands extremely large investments to build bigger and brighter instruments. Another approach, complementary but very powerful, is to perform small scale, low-energy experiments but with very high precision¹. In the last 20 years in particular, this led the community to look for new physics in much larger objects than elementary particle: molecules. Many proposals and experiments have demonstrated the potential of molecules to test our fundamental understanding of Nature: from probing variations of fundamental constants like the electron mass over the proton mass  or the fine-structure constant , or different models for dark matter, to testing the symmetrisation postulate of Quantum Mechanics or Bose-Einstein statistics. Especially notable are experiments currently responsible for the most stringent limit on the value of the electron electric dipole moment (which constraints strongly potential BSM theories).

However, the field is still in its infancy: the molecules that are currently used to explore new physics are composed of 2, 3 or 4 atoms at most, even though larger molecules offer qualitatively distinct opportunities in fundamental physics. The abundance of nearly degenerate rovibrational levels in the larger, polyatomic molecules (with more than 10 atoms) enhances their sensitivity to variations of fundamental constants or to potential ultralight bosonic dark matter. They can also provide a new system for implementing quantum computing, using either rotational or vibrational levels, and taking advantage of robust systematic error rejection schemes. Having more than 3 atoms also opens up the possibility to use molecular chirality to probe parity-violating interactions. Indeed, the energy levels of two enantiomers (the mirror-images of chiral molecules) are predicted to be slightly different, because of the parity-violation inherent to the weak interaction, or as a result of their interaction with dark-matter fields. Thus, a measurement of this symmetry-breaking energy difference ΔE_PV is a sensitive probe of the Standard Model and of physics BSM.

However, as the number of atoms in a molecule goes up, the number of energy levels increases very rapidly, as well as the couplings between these energy levels. These couplings limit the applicability of the usual cooling mechanisms because they induce population losses towards many different states. Couplings between different vibrational modes (denoted in the literature as Intramolecular Vibrational Redistribution (IVR)) also blur out spectroscopic resolution by what can be viewed as a loss of coherence due to non-radiative dynamic energy distribution in the molecule.

In this talk I will review the current use, the potential ahead and the obstacles on the way of using molecules for tests of fundamental physics, focussing on our on-going attempt to measure ΔE_PV in chiral molecules at the Laboratoire de Physique des Lasers.

Quantum Turbulence across Dimensions

Quantum turbulence (QT) is a disordered state of quantized vortices in superfluids and atomic Bose–Einstein condensates (BECs), offering a minimal and well-defined model of turbulence [1]. Its properties strongly depend on dimensionality.

1. Turbulence: 2D vs 3D
In three-dimensional (3D) classical and quantum turbulence, vortex stretching plays a central role, enabling a direct energy cascade from large to small scales (Richardson cascade), where energy is dissipated at short wavelengths. In contrast, two-dimensional (2D) turbulence lacks vortex stretching. Instead, both energy and enstrophy are conserved in the inviscid limit, leading to a dual-cascade scenario: an inverse energy cascade toward large scales and a direct enstrophy cascade toward small scales, as described by Kraichnan and Batchelor. The inverse cascade promotes the formation of large-scale coherent vortical structures, closely related to Onsager’s statistical theory of point vortices and negative-temperature states.

2. 2D Quantum Turbulence
In atomic BECs described by the Gross–Pitaevskii (GP) model, realizing ideal 2D turbulence is challenging. Vortex–antivortex pair annihilation breaks strict enstrophy conservation, compressibility introduces sound emission, and finite system size limits the inertial range. As a result, numerical and experimental studies often observe dominant direct cascades rather than clear inverse cascades. Nevertheless, under suitable conditions, vortex clustering and signatures of Onsager-like states have been reported, highlighting the interplay between vortex dynamics, dissipation, and finite-size effects in 2D QT.

3. Crossover across Dimensions
To clarify how QT changes across dimensions, we control the confinement height R₂ in a cylindrical box potential within the GP framework. For small R₂, the system shows 2D-like behavior: vortex–antivortex annihilation dominates and clustering consistent with inverse transfer develops. For large R₂, Kelvin waves can be excited along vortex lines, inducing reconnections, phonon emission, and a direct energy cascade characteristic of 3D Vinen turbulence. The results demonstrate that the excitation of Kelvin waves, determined by the system height, governs the transition from 2D to 3D QT.

References:

[1] M. Tsubota and K. Kasamatsu, Quantum Hydrodynamics and Turbulence, Oxford University Press (2025).
[2] L. Onsager, Nuovo Cimento Suppl. 6, 279 (1949).
[3] R. H. Kraichnan, Phys. Fluids 10, 1417 (1967).
[4] G. K. Batchelor, Phys. Fluids Suppl. 12, II-233 (1969).
[5] W.-C. Yang, X. Wang, and M. Tsubota, arXiv:2502.06133 (2025).

Title of seminar: Generalised hydrodynamics for integrable and quasi-integrable gases

Abstract:

I will review the theory of generalised hydrodynamics (GHD) for integrable quantum gases, with a particular emphasis on the impact of integrability-breaking perturbations and the emergence of long-range correlations during the dynamics even when integrability is broken.

Title of seminar:  Stages of the relaxation for a far-from-equilibrium trapped superfluid

Abstract:

Understanding the initial conditions that drive many-body quantum systems out of equilibrium is essential for getting into their thermalization dynamics. In this work [1,2,3], we identify two excitation regimes that lead a trapped Bose–Einstein condensate into turbulence regime evolving resulting in distinct final states. In the subcritical regime, the condensate partially reemerges after turbulence, whereas in the supercritical regime it dissolves completely into a thermal state. Despite these differences, both cases exhibit relaxation stages with common features: a direct energy cascade, the emergence of a nonthermal fixed point (characterized by identical scaling exponents), a prethermalization plateau, and eventual thermalization. Our results show that turbulence relaxation develops independently of the system’s initial conditions or its ultimate state. At late times, both regimes display universal scaling dynamics consistent with predictions from wave turbulence theory. Finally, by analyzing the time evolution of the condensate’s momentum distribution center of mass, we demonstrate how kinetic-energy dominance reflects the thermalized nature of the final states in each regime. The present results advance the field by showing that thermalization in far-from-equilibrium quantum systems follows universal relaxation dynamics, even when the final states differ, thereby deepening our understanding of the time evolution of many-body quantum systems. We shall present a new possible interpretation for pre-thermalization as well as an analysis of the construction of the coherent length during the last stage of thermalization of the system.

1. M. A. Moreno-Armijos et al – Phys. Rev. Lett. 134. 023401 (2025)
2. L. Madeira et al – Proc. Nat, Acad. Sci. -PNAS 121, 24044828121 (2024)
3. A. D. García-Orozco et al – Phys. Rev. A 106, 023314(2022)

Title of seminar: An optical lattice clock with a bosonic isotope of mercury

Abstract:

Among other neutral species, mercury has interesting properties for an optical lattice clock such as a low sensitivity to blackbody radiation (16 times lower than Yb, 30 times lower than Sr) and a
high vapour pressure at room temperature. So far, the 199Hg fermionic isotope was the only isotope used in a mercury clock, but its limited lifetime in the excited state will prevent to fully exploit the
new generation of ultrastable lasers to come. Using bosonic isotopes instead is a way to bypass this limit thanks to a potentially unlimited lifetime.

We report the first observation of the ¹⁹⁸Hg bosonic transition in an optical lattice clock, which results from several key experimental developments and a challenging search of a narrow transition in a wide uncertainty span.

The bosonic clock transition is forbidden and needs to be induced thanks to a high magnetic field. This is the so called quenching method¹. It allows longer probing times, adjustable to the laser properties. Hence, a first key step was developing a setup to produce a large enough magnetic field to induce the bosonic transition with the highest coupling achievable. Another challenge was implementing a widely tunable and flexible probe laser while maintaining the ultra-low noise properties, in order to probe any of the mercury isotopes with no additional noise. The coupling is also increasing with the probe power, so a major step was to significantly increase our deep UV ultrastable light power.

Given all these experimental advances, we calculated that we still had a relatively weak coupling and hence a narrow line transition to be found in a large frequency span. We performed various measurements and checks thanks to the ¹⁹⁹Hg isotope, especially a substantial alignment work, to optimize our chances to find the transition. Thanks to cumulated efforts, the search of the 198Hg transition was a success, making it the first observation of a mercury bosonic isotope transition.

We further obtained an operational optical lattice clock with the bosonic ¹⁹⁸Hg, already reaching a stability of 10-15 at 1 second. We have undertaken several studies of this new transition, in particular we measured the quadratic Zeeman shift coefficient with an uncertainty suitable to control this shift to 10^-17 or better. We also started studying other systematic effects such as the probe light shift and measuring the magic wavelength. Finally, we are working towards measuring, for the first time, the ¹⁹⁸Hg/⁸⁷Sr optical frequency ratio.

• ¹ A. V. Taichenachev et al., “Magnetic Field-Induced Spectroscopy of Forbidden Optical Transitions with Application
  to Lattice-Based Optical Atomic Clocks”, Phys. Rev. Lett., vol. 96(8), p.083001, 2006.

Title of seminar: Fast compression of a Bose-Einstein condensate and the new experiment for 2D Bose gases

Abstract:

In this seminar, I will first present the latest results on the evolution of a Bose-Einstein condensate after an abrupt compression where our preliminary analysis indicates an enhanced three-body losses.

Next, I will focus on describing a new experimental setup which aims to produce a 2D Bose gas of potassium-39 and explore the dynamics of quantum vortices in two dimensions.