Actualités & Séminaires

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.

Quantum hydrodynamics and turbulence in atomic BECs

Quantum hydrodynamics and turbulence [1] have been long studied in superfluid helium since 1950’s, and in atomic Bose-Einstein condensates (BECs) too. In this talk, I would discuss the important topics of atomic BECs.

Introduction

We will overview briefly characteristics of quantum hydrodynamics and turbulence compared with those of classical turbulence.

Single-component BECs

Turbulence likes isotropy [2] and rotation causes anisotropy. Such competition of two contradictory effects has been studied in classical turbulence [3]. Rotation in a quantum fluid forms a vortex lattice along the rotational axis and gives other interesting effects. There are not so many experimental works [4-6] on rotating quantum turbulence, and theoretical numerical studies are also limited [7]. I will discuss rotating quantum turbulence of BECs [8].

Two new scenarios for quantum friction: chiral media and rotating molecules

In the first scenario, we study the quantum friction experienced by a polarizable charged particle moving with parallel motion to a chiral medium-vacuum interface. We employ macroscopic quantum electrodynamics ^1,2 to obtain the Casimir–Polder frequency shift and decay rate. These results are a generalization of the respective quantities to matter being insensitive to parity². By examining the non-retarded and retarded limits we find that the an effective optical rotatory strength is relevant for a non-vanishing frequency shift and decay rate. For the second scenario, we investigate the rotational motion of diatomic molecules in free space interacting with the quantum electromagnetic field³. Using macroscopic quantum electrodynamics² we obtain the rotation-dependent decay rates of the molecule. By analyzing the behavior of the resulting rates at zero and finite temperature, we find a connection between the decelerating rotational dynamics and quantum friction.

¹ Stefan Yoshi Buhmann, David T. Butcher and Stefan Scheel. New Journal of Physics 14, 083034 (2012).
² S. Y. Buhmann. Dispersion Forces II. Many-Body Effects, Excited Atoms, Finite Temperature and Quantum Friction. (Springer, Berlin Heidelberg, 2012).
³ Stefan Yoshi Buhmann, M. R. Tarbutt, Stefan Scheel, and E. A. Hinds, Phys. Rev. A 78, 052901 (2008).

Non-stationary quantum many-body dynamics

Rydberg atoms in the vicinity of an optical nanofiber

Connecting atomic quantum registers via photonic channels into a quantum network is a promising way to achieve scalability, one of the crucial challenges in quantum technologies. Free-space scenarios considered so far are relatively easy to implement but suffer the drawback of strong losses. Optical nanowaveguides constitute an interesting alternative which offer strong transverse confinement of the field and hence strong coupling to atomic nodes in their vicinity. In particular optical nanofibers (ONFs) received much attention within the past two decades.

On the other hand, within the past two decades, the giant van der Waals interactions between Rydberg atoms and the associated blockade phenomenon have been key in many quantum information proposals, including atomic quantum registers and repeaters. In free-space, the energy shifts induced by the van der Waals interaction between two atoms prepared in levels of principal quantum numbers n > 50 and a few µm apart can be of the order of tens of GHz and effectively forbid their simultaneous resonant laser excitation. This blockade mechanism was employed to generate entanglement and perform gates in systems of individual atoms and atomic ensembles.

In this seminar, I shall present the theoretical results my collaborators and I obtained on the spontaneous emission and van der Waals interaction of rubidium atoms in the vicinity of a silica ONF [1,2] in view of building a Rydberg-atom-ONF interface. In particular, on the specific cases of S and P states, I shall analyze how the presence of the nanofiber and the symmetry breaking it induces modify the blockade and the anisotropy of the Rydberg-Rydberg interaction.

References

[1] E. Stourm, M. Lepers, J. Robert, S. Nic Chormaic, K. Mølmer, E. Brion, Spontaneous emission and energy shifts of a Rydberg rubidium atom close to an optical nanofiber, Phys. Rev. A 101, 052508 (2020).
[2] E. Stourm, M. Lepers, J. Robert, S. Nic Chormaic, K. Mølmer, E. Brion, Interaction of two Rydberg atoms in the vicinity of an optical nanofibre, New J. Phys. 25, 023022 (2023)

Quantum-Noise Reduction to improve Gravitational-Wave Detectors

Detection of gravitational waves by the Virgo detector requires exquisite control of all noise sources. In particular, a world-leading short-term frequency stability of the laser circulating in the interferometer on the mHz range is required for Virgo’s performance target [1].

The CALVA experiment at IJCLab is designed to study quantum states of light to improve Virgo’s performance. In particular, we are testing a method based on a Fabry-Perot cavity (filter cavity) for producing frequency-dependent squeezed states of light, which hold great promise for the future of gravitational-wave detectors [2]. This test requires excellent control of the laser frequency, at a level that would be compatible with Virgo’s performance. For this, we will use the ultra-stable frequency reference from the research infrastructure REFIMEVE, of which IJCLab is a member, and which is already available in the laboratory [3]. Because our laser has a different wavelength from the REFIMEVE signal (1064 nm vs. 1542 nm), an optical frequency comb is needed to transfer the stability of the ultra-stable signal to our laser.

The recent acquisition of an optical frequency comb by our team will enable a stability in the Hz range, improving our current stability by 3 orders of magnitude. This will help keep the CALVA experiment suitable for upcoming generations of gravitational-wave detectors, such as the Einstein Telescope project, not only in terms of performance, but also in terms of adaptability of the operating wavelength.

[1] F. Bondu, P. Fritschel, C. N. Man, and A. Brillet, Opt. Lett. 21, 582-584 (1996).
[2] C. M. Caves, Phys. Rev. D, 23:1693-1708 (1981).
[3] E. Cantin, M. Tønnes, R. L. Targat, A. Amy-Klein, O. Lopez, et P.-E. Pottie, New J. Phys. 23 (2021)

Effet Casimir Dynamique avec des atomes: de l’émission de paires de photons à une phase Sagnac quantique

L’interaction entre la matière et le champ électromagnétique quantique est particulièrement intrigante lorsque les effets de mouvement entrent en jeu. L’effet Casimir dynamique (ECD) est généralement considéré dans une approche macroscopique basée sur les conditions aux limites [1] pour le champ quantique. À un niveau microscopique plus fondamental, l’ECD peut être décrit à l’échelle atomique à l’aide de modèles hamiltoniens d’optique quantique pour le couplage atome-champ. Une observation expérimentale d’un analogue de l’ECD a été rapportée avec des supraconducteurs. Néanmoins, une mesure directe du rayonnement ECD produit par des corps en mouvement reste hors de portée car ce rayonnement est extrêmement faible dans des conditions expérimentales réalistes. Une voie alternative consiste à suivre des signatures indirectes d’aspects analogues à l’ECD dans les interféromètres atomiques, produites par le mouvement relatif entre l’atome et une surface conductrice. Dans cette présentation, nous présentons une théorie microscopique de l’effet Casimir dynamique [2,3], et utilisons ce formalisme pour prédire des effets analogues à l’ECD dans les interféromètres atomiques [4,5]. Notre approche révèle des connexions entre des aspects non triviaux de la physique moderne tels que la retardation électrodynamique, les phases géométriques, la non-localité et les effets non-inertiels.

La diffraction d’atomes rapides au dessus d’une surface cristalline

Découverte, il y a presque vingt ans la diffraction d’atomes légers entre 200 eV et 5 keV au dessus d’une surface trouve des applications pour le suivie de la croissance directement en bâti d’épitaxie ou elle permet un diagnostique fin du mode de croissance et de la topologie/reconstruction de la dernière couche. Je présenterai les évolutions récentes sur les processus inélastiques et les mesures des forces attractives vers la surface.

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Diffraction of fast atoms above a crystalline surface.

Discovered almost twenty years ago, the diffraction of light atoms between 200 eV and 5 keV above a surface is finding applications in the monitoring of growth directly in the epitaxial frame, where it enables fine diagnosis of the growth mode and the topology/reconstruction of the last layer. I will present recent developments in inelastic processes and measurements of attractive forces towards the surface.

Laser absorption with non-resonance cavities and Faraday rotation spectroscopy for gas sensing applications, towards laboratory detection of OH and HO2 radicals.

Laser absorption spectroscopy is a fundamental technique widely used in trace gas sensing, with versatile applications in atmospheric chemistry, environmental monitoring, biomedical diagnostics, and molecular astrophysics. By quantifying the attenuation of light passing through a gas sample, this non-invasive method enables in-situ and absolute quantification of sample concentration. The utilization of optical cavities can significantly increase the light-sample interaction length, thereby enhancing detection limits according to the Beer-Lambert law. Among laser-based cavity techniques, off-axis Cavity-Enhanced Absorption Spectroscopy (off-axis CEAS) stands out as a robust approach for trace gas detection. A key feature of this approach involves eliminating the cavity mode structure through the un-resonance coupling of a narrow bandwidth laser source into a cavity, facilitating continuous scanning of laser frequency for rapid spectral measurements. This strategy offers simple optical alignment and circumvents the need for high-bandwidth devices or laser frequency-locking, ensuring robustness against misalignment and mechanical vibration.

In this work, we present the developments and applications of non-resonance cavities for the direct quantification of OH and HO2 radicals, key species in atmospheric chemistry. Furthermore, the enhancement of optical effective path length not only amplifies the absorption of the target molecules but also extends to common atmospheric species such as water vapor and CO2. The occurrence of spectral interference, resulting from the overlapping absorptions of various species, poses challenges in accurately determining the concentration of the species of interest. To address these challenges, we introduce Faraday Rotation Spectroscopy, leveraging the magneto-optic effect observed only for paramagnetic species (e.g., NO, NO2, OH, HO2) to facilitate interference-free measurements. The combination of the optical cavity and Faraday rotation technique enhances both sensitivity and selectivity for gas concentration determination.

Earth – Moon – Mars crustal attenuation properties comparison

Following its deployement at the surface of Mars, the SEIS seismometer of the NASA-InSight mission recorded tens of high-frequency Martian seismic events (> 1Hz) which we analyzed to characterize the attenuation properties of the Martian lithosphere from an Earth-Moon-Mars comparison perspective. The Martian waveforms are generally depolarized and show P and S arrivals with a gradual beginning, a broad maximum and a very long coda decay. These characteristics are highly reminiscent of the seismic wavefield in the terrestrial oceanic lithosphere at high frequency (Po and So above 2Hz), where the stratification of the absorption was used to estimate the Asthenosphere-lithosphere boundary (Takeuchi et al., 2017). To constrain the attenuation properties on Mars, we modeled the energy envelopes of the Martian events using a multiple-scattering approach in which we considered a stratification of the velocity and the attenuation properties in the medium. We observed that a simple model composed of a highly scattered crust and a weakly inhomogeneous mantle is sufficient to explain Martian events. We found that the Martian crustal diffusivity (10-12 km²/s) is similar to the estimation obtained in the lithosphere of the Atlantic Ocean (15–60 km²/s, Hannemann et al. 2022), but higher than the Lunar crust value (2 km²/s). The absorption attenuation results indicate that the Martian crust is globally dry ~ 10^-4 compared to the terrestrial crust ~ 10^-3. Our results suggest that the basaltic nature and the heterogeneities of the crust are the main source of the scattering in the Martian lithosphere unlike the extend of the fractures with depth observed on the Moon.

Experimental strategies to track conformational changes in the gas phase

Complex molecular ions, like synthetic polymers or biomolecules, also display complex conformational landscapes, in relation with their hierarchical structuration. Characterizing such complex landscapes also poses experimental challenges. Gas phase approaches, based on mass spectrometry allow some degree of simplification, by isolating species of well-defined composition and environment. However, gas-phase structure-sensitive techniques, often based on optical spectroscopy, are not easy to apply to large, flexible polymers. In this context, ion mobility spectrometry (IMS) is an interesting compromise, as a low-resolution structural probe applicable to large (bio-)molecular species. Indeed, the “collision cross sections” (CCS) determined by IMS-MS have been widely used to propose structures for protein complexes. Our group has proposed several strategies to exploit IMS-MS beyond CCS measurements. In particular we demonstrated that tandem IMS measurements could be used to explore the (photo-)isomerization of complex molecular systems. More recently, we developed approaches to follow the kinetics of conformational changes in such systems, and derive the associated thermodynamical quantities. This seminar will describe the principle of tandem ion mobility measurements, and give an overview of the approaches developed in our lab.

Rydberg excitons : a nonlinear optics perspective

For the last decade, highly excited (Rydberg) states of electron-hole bound states (excitons) in the semiconductor Cu2O have been showing more and more similarities with their atomic relatives, enabling solid-state Rydberg physics. After a rapid overview on Cu2O Rydberg excitons, I will focus on two recent experimental results where Rydberg-induced optical nonlinearities are central. In the first case, the giant interactions between Rydberg states are used to produce a large self-Kerr effect. This is directly observed with spatially-resolved interferometric imaging. In a second example, I will discuss very recent result where a second harmonic generation process is time-resolved with sub-picosecond resolution to directly unveil the Rydberg states lifetime along with several other intriguing dynamics.

Fiber based high power lasers for quantum computing applications

Observation of relaxation stages in out-of-equilibrium closed quantum systems

The temporal evolution of a non-equilibrium quantum system is the most crucial aspect of its dynamics. We introduce ro-translational excitations in an 87Rb Bose-Einstein condensate, which, under certain conditions, induces turbulence in the system . Once this non-equilibrium state is achieved, the system is allowed to evolve until it reaches equilibrium. Throughout this dynamical process, the characteristic particle cascade is observed, featured by the appearance of a power-law in the momentum distribution along with temporal windows of meta-stability showing the emergence of universal scaling. Subsequently, the system reaches a pre-thermalization stage with minimal variation, followed by a reverse cascade that forms a new condensate with fraction and temperature different from the initial one. This allows the assessment of various energy inputs during relaxation. The system is investigated under different initial excitations and represents a closed system suitable for exploring new physics related to the relaxation dynamics of non-equilibrium quantum systems.