The emerging generation of optical clocks holds great perspectives for fundamental physics and open up new fields of applications, such as chronometric geodesy. These clocks, reaching residual frequencies instabilities in the low $10^{-18}$, require means of comparison in the optical frequency domain. Mainly optical fiber link can reach sufficient performance, but are the limiting factor for applications in need of reconfigurable, rapidly deployable or in space links. Our work aims to demonstrate a ground-to-ground stabilized free-space optical link via an airborne relay. We are currently working on a 600 m folded free space link, achieving stability of $10^{-18}$ after 20 s of integration. I will present our link design and results as well as development perspectives toward our end goal.

Ryderg atoms offer an ideal platform for the study of long-range interacting systems. However usual techniques for imaging and cooling are unavailable in alkali Rydberg atoms. Our approach rely on the use of a two-optically-active-valence-electron atom such as ytterbium. Ionic core transitions of this atom offer new perspectives for optical manipulation in the Rydberg state. I will present work on the isolated core excitation of ultra cold ytterbium Rydberg atoms of high orbital quantum number. The extracted energy shifts and autoionization rates are in relatively good agreement with a model based on independent electrons taking into account interactions with a perturbative approach.

In the context of the improvement of the Advanced Virgo gravitational wave detector, the quantum noise contribution to the detector noise has to be reduced in order to increase its sensitivity and consequently the observable volume of the Universe. One of the idea to go beyond the Standard Quantum Limit is to use frequency dependent squeezed states of light. The implementation of this technique is tested on the CALVA experiment at IJCLab in the framework of the Exsqueez ANR in collaboration with LKB, IP2I and LAPP. In this presentation, I will give the basis of gravitational wave detection, quantum noise and squeezing to present the design of the experiment done at IJCLab followed by the characterization results of the first optical systems used to produce and measure frequency independent squeezing, a first step for the obtention of frequency dependent squeezing.

As soon as the concept of matter-wave duality rose from the early development of quantum mechanics, the possibility of creating atomic interferometers has been studied. Measurement of rotation rates through the Sagnac effect, well known in optics, became possible with atomic waves around 1990. Nowadays, cold-atom gyroscopes can reach high sensitivities competing with optical Sagnac interferometers, like fiber gyroscopes. Cold-atom inertial sensors feature promising applications in navigation, geoscience and for tests of fundamental physics. In our experiment, we laser-cool cesium atoms to a temperature of 2.0 $\mu$K and launch them vertically at a velocity of 5 m$\cdot$s$^{-1}$. Light pulse atom interferometry with counter propagating Raman transitions is used to create an interferometer with a Sagnac area of 11 cm$^2$. We then detect the internal state of the atoms at the end of the interferometer using fluorescence detection. The SYRTE cold atom gyroscope represent the state-of-the-art of atomic gyroscopes with a long term stability$^1$ of 3$\cdot10^{-10}$ rad$\cdot$s-1. The gyroscope has been used to test new methods to reach better sensitivity, like the possibility to work without dead time by interrogating three atomic clouds simultaneously$^2$, allowing us to reach a sampling rate of 3.75Hz. To reach such stability, we need to understand and minimize the systematic effects, the main one coming from the coupling of an imperfect launch velocity and a misalignment between the two Raman beams used to perform the interferometer$^3$. In this talk I will present our work on the evaluation of the scale factor of the gyroscope and how it allows us to test the validity of the Sagnac effect for matter waves. The phase shift induced by Earth rotation depends on the angle between the oriented Sagnac area of the interferometer and the geographic north. By rotating our apparatus, we are able to vary this angle, and therefore modulate the phase shift. This allows us to perform a test of the Sagnac effect with a relative accuracy of 2$\cdot$10$^{-4}$, which represents an improvement of a factor 100 compared to previous matter wave experiments. 1 “Interleaved Atom Interferometry for High Sensitivity Inertial Measurements” D. Savoie, M. Altorio, B. Fang, L. A. Sidorenkov, R. Geiger, A. Landragin, Science Advances, Vol. 4, no. 12, eaau7948 (2018) 2 “Continuous Cold-Atom Inertial Sensor with 1 nrad/sec Rotation Stability” I. Dutta, D. Savoie, B. Fang, B. Venon, C. L. Garrido Alzar, R. Geiger, and A. Landragin, Phys. Rev. Lett. 116, 183003 (2016) 3 “Accurate trajectory alignment in cold-atom interferometers with separated laser beams” M. Altorio, L. A. Sidorenkov, R. Gautier, D. Savoie, A. Landragin and R. Geiger, Phys. Rev. A 101, 033606 (2020)

The two Strontium OLC developed at SYRTE have shown uncertainty in the low $10^{-17}$, confirmed within local and international comparisons against both microwave and optical frequency reference. Improving their performance require better control and understanding of systematic effects. We present a new generation experimental chamber, under assembly and to be installed on Sr2 system, which is expected to enable a one order of magnitude reduction on the BBR uncertainty, predominant on the discussed system. We also present measurement of the AC Stark shift of the clock transition, with an investigation dependence of the atomic polarizabilities with the polarisation of the lattice light.

Two-way satellite time and frequency transfer (TWSTFT) is a technique used on a regular base to compare atomic clocks and local time scales of laboratories all over the world. However, its instability is limited mainly due to the modulation bandwidth of the signal. In the framework of the EMRP project “ITOC” (International Time scales with Optical Clocks), a unique measurement campaign was carried out, exceeding these limits by using the maximum bandwidth available for this technique so far for the first time. Within this campaign, five optical clocks and six atomic fountain clocks located in INRIM, LNE-SYRTE, NPL and PTB had been compared simultaneously over a duration of 26 days. GPS Precise Point Positioning had been used in parallel as a second, independent satellite-based technique for comparison. By applying an analysis procedure taking into account gaps and correlations on the data, results in the low 10$^{-16}$ uncertainty range could be obtained. The presentation will review the basic concept of broadband TWSTFT, show the challenges of such a campaign and give details on the data analysis.

Ultra-stable laser is an important component of optical lattice clocks which are candidates for the future redefinition of the SI second. Spectral hole burning in rare earth doped crystals can provide narrow atomic transitions based on the absorption of optical frequencies by the doping ions. Ultra-stable lasers achieved by frequency locking a laser radiation on such narrow atomic transitions are expected to exhibit lower fundamental (thermal noise induced) limits compared to traditional system utilizing high finesse Fabry-Perot cavity, due to operation at cryogenic temperature and high mechanical quality factor of the crystal, compared to the amorphous glasses typically used in conventional Fabry-Perot Cavity designs. In our laboratory, we are using the europium doped yttrium ortho-silicate crystal (Eu:YSO), at cryogenic temperature (below 4 K). We have recently reported a double heterodyne detection regime which demonstrates a detection noise compatible with 4x10$^{-16}$ fractional frequency stability at 1s, and we have effectively demonstrated laser fractional frequency stability at 1.7 x 10$^{-15}$ at 1s. We have experimentally evaluated mechanical-deformation-induced frequency shifts of the spectral holes in Eu:YSO, which allows deducing the acceleration sensitivity of the setup, and, in the future, optimizing the design of the crystal mount. We have also quantitatively studied the effect of externally applied electric fields. Furthermore, the sensitivity of the resonant frequency to temperature changes was evaluated. A special environment where the crystal is staying in a cold He gas-filled chamber was implemented. In this environment, temperature changes induce a change of He pressure applied to the crystal. Temperature changes therefore modify the resonant frequency by two processes: direct temperature-sensitivity-induced shift, and, in parallel, pressure-change-induced shift. For a “magic” pressure-temperature couple these two processes can result in a first-order global cancellation of the temperature sensitivity of the frequency of the spectral hole which we have recently shown experimentally.

Individual atoms immersed into a superfluid form a paradigm of quantum physics. It lies at the heart of many models exploiting the quantum nature of individual atoms to understand quantum phenomena or to open novel routes to local probing and engineering of quantum many-body systems. In the talk, I will present our approach of controlled immersion of individual, localized neutral Cesium (Cs) atoms into a Rubidium (Rb) ultra-cold bath [1]. The first part of my talk is dedicated to local probing: I will present our experimental realization of single-atom quantum probes for local thermometry based on the spin dynamic of the Cs atoms immersed into the Rb ultracold gas. By controlling microscopic atomic collisions, we map thermal information about the gas onto the quasi-spin population of the probe. Our probe is not restricted to measure temperature, but it allows sensing any mechanism affecting the total collisional energy in a spin-exchange collision such as the magnetic field, realizing also local magnetometry. In particular, I will show that having access to the dynamics of the microscopic process of motion-spin mapping allows us optimizing the information flow. Quantifying the sensitivity of our probe by the Quantum Fisher information, we find that it can outperform the steady-state limits setting the Cramér-Rao bound by roughly one order of magnitude [2]. The second part of my talk will be focused on the realization of a heat engine where the working fluid is embodied by the Cs atoms and the bath by the cloud of ultracold Rb atoms. Specifically, our engine is based on the quantum Otto cycle. Heat transfer is realized by spin-exchange collisions between the working fluid and the bath, while work is performed by changing the energy-level spacing of the engine with an external magnetic field. Thanks to the ability to follow the populations of individual atomic levels of a cesium atom in real time, we have performed precise characterization of the engine including the fluctuations of its power [3]. [1] F. Schmidt et al, Phys. Rev. Lett. 121, 130403 (2018). [2] Q. Bouton et al., Phys. Rev. X 10, 011018 (2020). [3] Q. Bouton, arXiv:2009.10946 (2020).

Many times people are so focused on physical signals that they loose sight of the information they are really interested in. This is the heritage of the past, where processing was analog and each step necessarily implied a regeneration of the physical signals. Even now, that digital electronics is well established since decades, this forma-mentis survives and represents a real limitation in taking full advantage of digital electronics and to really extend the beneficial effects of the digital revolution to time and frequency metrology. The seminar will focus on timescales generation as a case study to show the differences between the two approaches.

In the standard current cosmological model cold dark matter (DM) accounts for about 23% of the energy of the universe and about 83 % of it's matter content. It is postulated in order to explain numerous astronomical and cosmological observations, and is assumed to exist e.g. in our galaxy and in our solar system. In ~2015 and the following years first publications described the possibility that dark matter fields might introduce local oscillations or transient variations of fundamental constants that could be measured by atomic clocks and/or ultrastable oscillators. I will describe some experiments and data analysis of searches for such DM at SYRTE and in collaborations involving SYRTE, that took place between 2015 and today and are partly still ongoing, unfortunately with no positive detection event so far. For the sake of keeping in time, I will concentrate on oscillating DM, and only briefly mention transient DM searches.

Séminaire exceptionnel en langue Française sur la thématique "Open Data" dans le domaine de la recherche Cécile ARÈNES Chargée de mission Données de la recherche et Humanités numériques Bibliothèque de Sorbonne Université

La Bibliothèque de l’Observatoire de Paris a créé un portail HAL pour répondre aux attentes du MESRI et aux obligations de dépôt pour les publiants du CNRS, les porteurs de projets ANR et européens. Ce portail a déjà été alimenté avec les notices des publications des chercheurs des différents laboratoires de l’Observatoire de Paris. A ce jour, cela représente l’ensemble des publications présentes sur ADS et parues dans des revues à comité de lecture entre 2012 et 2020. Pour répondre aux recommandations du MESRI et du CNRS, la Bibliothèque de l’Observatoire de Paris souhaiterait développer avec le SYRTE sa politique de libre accès afin de faciliter le travail des chercheurs et ingénieurs du laboratoire. La collection du SYRTE dans le portail HAL de l’Observatoire comptabilise d’ores et déjà 460 documents (avec dépôt du texte intégral) et 562 notices – références bibliographiques. La bibliothèque ayant déjà mené plusieurs séances de présentation de HAL dans d'autres laboratoires propose de présenter aux membres du SYRTE le fonctionnement et les possibilités offertes par les archives ouvertes HAL. Cette presentation permettra également d’avoir un échanges sur ses aspects pratiques. SEMINAIRE EN FRANCAIS

The emerging generation of optical clocks holds great perspectives for fundamental physics and open up new fields of applications, such as chronometric geodesy. These clocks, reaching residual frequencies instabilities in the low $10^{-18}$, require means of comparison in the optical frequency domain. Mainly optical fiber link can reach sufficient performance, but are the limiting factor for applications in need of reconfigurable, rapidly deployable or in space links. Our work aims to demonstrate a ground-to-ground stabilized free-space optical link via an airborne relay. We are currently working on a 600 m folded free space link, achieving stability of $10^{-18}$ after 20 s of integration. I will present our link design and results as well as development perspectives toward our end goal.