Title of seminar: UV-Dual Comb Spectroscopy using a bidirectional mode-locked femtosecond Ti:Sapphire ring laser

Abstract :

The development of broadband, precise, and rapid molecular spectroscopy addresses current challenges in gas detection within dynamic and complex environments. I will present dual-comb spectroscopy, a technique that is particularly well suited to meeting these challenges. At the Institute of Light and Matter (Lyon), we are developing dual-comb spectroscopy in the UV spectral range to probe highly reactive species such as radicals. I will introduce the experimental dual-comb source we have developed, based on a mode-locked femtosecond Ti ring laser, discuss its unique characteristics and present spectroscopic results obtained in both the IR and UV spectral regions.

Title of seminar: Tests of fundamental physics using ytterbium ions

Abstract :

I will present the first results from my postdoc at PTB in Braunschweig, Germany. Trapped ions are widely used in optical clocks and fundamental physics. Among them, the forbidden electric octupole (E3) transition in 173Yb⁺ offers key advantages, including an extremely narrow natural linewidth and high sensitivity to physics beyond the Standard Model. The large electric quadrupole moment of its deformed nucleus is predicted to enhance this forbidden transition. I will show how we confirmed this effect and observed hyperfine-state-dependent quenching, paving the way toward multi-ion clocks using 173Yb⁺. I will further show how the rich hyperfine structure can be used to probe the nuclear properties of 173Yb⁺, making it a particularly promising candidate for searches of a fifth force using isotope shifts measurements and King plot analysis.

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.

Title of seminar: Forming and deforming dipolar supersolids under dipole tilt – from triangular droplet arrays to stripes

Abstract :

Gases of highly dipolar atoms exhibit unique interaction properties that give rise to striking many-body phenomena, both at the mean-field level and beyond [1]. About a decade ago, a universal stabilization mechanism driven by quantum fluctuations was discovered in these systems. This mechanism prevents collapse and enables the emergence of exotic states of matter, including ultradilute quantum droplets, crystallized quantum states, and – most notably – supersolids, which combine the seemingly antithetical properties of superfluids and solids [2].

In my talk, I will present our ongoing experimental and theoretical work on quantum phase transitions in dipolar quantum gases [3]. Supersolid states with distinct two-dimensional lattice structures are formed from initially uniform superfluids in anisotropic traps through magnetic-field ramps and rotations, tuning the interaction strength and the dipole orientation, respectively. A structural rearrangement of the state occurs through magnetic-field rotation, marked by a deformation of the triangular lattice arrangement observed for perpendicular magnetization, and the transition to stripe supersolid states, as well as potential square supersolids. At intermediate angle, the system exhibits bistability between these configurations, indicating metastable states and a potential first-order quantum phase transition.

References
[1] L. Chomaz et al., « Dipolar physics: A review of experiments with magnetic quantum gases, » Reports on Progress in Physics 86, 026401 (2023)
[2] L. Chomaz, « Quantum-stabilized states in magnetic dipolar quantum gases. » Annual Review of Condensed Matter Physics17 (2025)
[3] K. Chandrashekara et al., in preparation (2026)