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Personne# Victor Youri Helson

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Publications associées (9)

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Gaz de Fermi

Un gaz de Fermi idéal est un état de la matière constitué d'un ensemble de nombreux fermions sans interaction. Les fermions sont des particules ayant un spin demi-entier (1/2, 3/2), comme les électr

Matière

En physique, la matière est ce qui compose tout corps (objet ayant une réalité spatiale et massique). C'est-à-dire plus simplement une substance matérielle et donc occupe de l'esp

Cavity quantum electrodynamics

Cavity quantum electrodynamics (cavity QED) is the study of the interaction between light confined in a reflective cavity and atoms or other particles, under conditions where the quantum nature of pho

This thesis reports on the realization of the first experiments conducted with superfluid, strongly interacting Fermi gases of 6Li coupled to the light field of an optical cavity. In the scope of existing ultracold atomic platforms, this is the first time that a system with strong ground state fermionic correlations is operated in the framework of cavity quantum electrodynamics (cQED).From a condensed matter perspective, the system features a fully controllable microscopic Hamiltonian with control over both the strength of the ground state and light-matter interactions and the geometry of the latter. This contrasts with usual solid-state systems, in which the properties of the ground states are hardly tunable. As such our experiment is the perfect platform to simulate the physics of strongly correlated matter coupled to light fields. The manuscript is divided in three parts. The first part is dedicated to the presentation of technical details of the experiment and of measurement techniques we routinely employ to create and probe our strongly interacting gases coupled to light. We introduce the use of the cavity as a probing tool by presenting the achievement of strong light-matter coupling between the atomic ensemble and the cavity field. Similarly, we present a robust thermometry technique for the unitary Fermi gas, with which we measure temperature of the gases deep in the superfluid, quantum degenerate regime.In a second part we focus on the measurements of the strong atom-atom correlations which emerge from the energy spectrum of the atoms-cavity system. We start by laying down the theoretical basis needed for the understanding of the origin of the strong atom-atom interaction, and present its consequence on the many-body wavefunction of the gas by introducing the two-body contact as a universal thermodynamic quantity. We then report on the observation of strong light-matter coupling between pairs of atoms and the cavity field via photoassociation transitions. We describe the resulting light-matter coupling strength in terms of the two-body contact, imprinting many-body correlations onto cavity spectra for the first time. In the following experiment we study the optomechanical response of the gas in the dispersive regime. We observe distorted cavity transmission profiles, signatures of the nonlinear Kerr effect. The strength of the nonlinearity is governed by the density response of the gas, which we express via an operator product expansion in terms of the contact.In the last part, we investigate the effects of engineering long-range, photon-mediated interactions in the gas. We formally show how the system is expected, above a critical value for the strength of the long-range interaction, to undergo a phase transition to a density ordered state. The onset of density-ordering is observed by the superradiant properties of the ordered phase, and we show that it is also controlled by the density response of the unperturbed gas. In addition, we measure the divergence of the density wave susceptibility as the strength of the long-range interactions approaches the critical points: a striking feature of phase transitions. By measuring its temperature after the experiment, we prove that the gas remains superfluid.

Jean-Philippe Brantut, Victor Youri Helson, Hideki Konishi, Kevin Etienne Robert Roux, Timo Zwettler

We study a Fermi gas with strong, tunable interactions dispersively coupled to a high-finesse cavity. Upon probing the system along the cavity axis, we observe a strong optomechanical Kerr nonlinearity originating from the density response of the gas to the intracavity field and measure it as a function of interaction strength. We find that the zero-frequency density response function of the Fermi gas increases by a factor of two from the Bardeen-Cooper-Schrieffer to the Bose-Einstein condensate regime. The results are in quantitative agreement with a theory based on operator-product expansion, expressing the density response in terms of universal functions of the interactions, the contact, and the internal energy of the gas. This provides an example of a driven-dissipative, strongly correlated system with a strong nonlinear response, opening up perspectives for the sensing of weak perturbations or inducing long-range interactions in Fermi gases.

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A density wave (DW) is a fundamental type of long-range order in quantum matter tied to self-organization into a crystalline structure. The interplay of DW order with superfluidity can lead to complex scenarios that pose a great challenge to theoretical analysis. In the past decades, tunable quantum Fermi gases have served as model systems for exploring the physics of strongly interacting fermions, including most notably magnetic ordering(1), pairing and superfluidity(2), and the crossover from a Bardeen-Cooper-Schrieffer superfluid to a Bose-Einstein condensate(3). Here, we realize a Fermi gas featuring both strong, tunable contact interactions and photon-mediated, spatially structured long-range interactions in a transversely driven high-finesse optical cavity. Above a critical long-range interaction strength, DW order is stabilized in the system, which we identify via its superradiant light-scattering properties. We quantitatively measure the variation of the onset of DW order as the contact interaction is varied across the Bardeen-Cooper-Schrieffer superfluid and Bose-Einstein condensate crossover, in qualitative agreement with a mean-field theory. The atomic DW susceptibility varies over an order of magnitude upon tuning the strength and the sign of the long-range interactions below the self-ordering threshold, demonstrating independent and simultaneous control over the contact and long-range interactions. Therefore, our experimental setup provides a fully tunable and microscopically controllable platform for the experimental study of the interplay of superfluidity and DW order.