Le travail présenté dans ce manuscrit porte sur l'avancement de l'expérience FORCA-G (FORce de CAsimir et Gravitation à courte distance) dont le but est la mesure par interférométrie atomique de ...forces à courte distance entre un atome, piégé dans un réseau optique vertical, et une surface. Réalisée à l'aide de transitions Raman stimulées, la séparation spatiale et cohérente des paquets d'onde atomique sur des puits adjacents du réseau permet de mesurer, après recombinaison, la différence d'énergie entre ces puits, liée à l'incrément d'énergie potentielle de pesanteur : la fréquence de Bloch nB. Pour de faibles densités atomiques, il est démontré une sensibilité court terme à 1 s de dn/nB = 1,8.10-6 à l'état de l'art des capteurs de forces à atomes piégés. La mise en place d'un système de refroidissement évaporatif, afin d'augmenter le nombre d'atomes par puits, permet désormais d'explorer des régimes de fortes densités atomiques où les interactions ne peuvent être négligées. Pour des densités de 1011-1012 at/cm3, il est montré qu'un phénomène d'auto-synchronisation des spins entre en compétition avec le mécanisme d'écho de spin. L'impact de ce phénomène sur le contraste et la fréquence mesurée est étudié dans un interféromètre où les deux paquets d'onde occupent le même puits. Des premières mesures sont ensuite effectuées dans le régime où les paquets d'onde sont séparés. Elles montrent un comportement différent qui reste à modéliser. Enfin, il est montré que le protocole de mesure permet de s'affranchir des biais collisionnels : les interactions atomiques limitent la sensibilité du capteur de force sans limiter son exactitude.
This thesis presents the recent progress on the FORCA-G (FORce de CAsimir et Gravitation à courte distance) experiment which aims at measuring short range forces between an atom, trapped in a vertical optical lattice, and a mirror. Stimulated Raman transitions are used to induce coherent transport between adjacent lattice sites to perform atom interferometry in order to measure with very high sensitivity, shifts in the Bloch frequency nu_B, which is the potential increment between two lattice sites. For low atomic densities, we demonstrate a local force sensor with state-of-the art relative sensitivity on the Bloch frequency of deltanu/nu_B= 1.8x10-6 at 1 s. The recent use of evaporative cooling, in order to increase the number of atoms per well, allows to work the experiment with much denser atomic clouds where atom interactions cannot be neglected. At densities of 1011-1012 at/cm3, it is shown that a spin self-rephasing mechanism competes with the spin-echo technique. The impact of the former mechanism onto the contrast and the measured frequency is studied in an interferometer where the two partial wave packets perfectly overlap. First measurements are then performed in a regime where the two partial wave packets are spatially separated. They show a different behaviour that remains to be modelled. Finally, it is shown that the measurement protocol allows to greatly reduce collisional shifts: atom interactions limit the sensitivity of the local force sensor without limiting its accuracy.
We demonstrate a light-pulse atom interferometer based on the diffraction of free-falling atoms by a picosecond frequency-comb laser. More specifically, we coherently split and recombine wave packets ...of cold \(^{87}\)Rb atoms by driving stimulated Raman transitions between the \(|5s~^2S_{1/2},F=1\rangle\) and \(|5s~^2S_{1/2},F=2\rangle\) hyperfine states, using two trains of picosecond pulses in a counter-propagating geometry. We study the impact of the pulses length as well as of the interrogation time onto the contrast of the atom interferometer. Our experimental data are well reproduced by a numerical simulation based on an effective coupling which depends on the overlap between the pulses and the atomic cloud. These results pave the way for extending light-pulse interferometry to transitions in other spectral regions and therefore to other species, for new possibilities in metrology, sensing of gravito-inertial effects and tests of fundamental physics.
We reflect on the prospect of exploiting the recoil associated with absorption and emission of photons to perform spectroscopy of a single molecular ion. For this recoil to be detectable, the ...molecular ion is sympathetically cooled by a laser-cooled atomic ion to near their common quantum mechanical ground state within a trapping potential. More specifically, we present a general framework for simulating the expected photon recoil spectra in regimes where either the natural transition linewidth \(\Gamma_t\) of the molecular ion or the spectral width \(\Gamma_L\) of the exciting light source exceeds the motional frequencies of the two-ion system. To exemplify the framework, we present two complementary cases: spectroscopy of the broad 3s \(^2\)S\(_{1/2}\) - 3p \(^2\)P\(_{3/2}\) electronic transition (\(\Gamma_t/2\pi = 41.8\) MHz) of a single \(^{24}\)Mg\(^+\) ion at \(\lambda=279.6\) nm by a narrow laser source (\(\Gamma_L/2\pi \lesssim 1\) MHz) and mid-infrared vibrational spectroscopy of the very narrow \(|v=0,J=1\rangle\) - \(|v'=1,J'=0\rangle\) transition (\(\Gamma_t/2\pi = 2.50 \) Hz) at \(\lambda=6.17\) \(\mu\)m in the \(^1\Sigma^+\) electronic ground state of \(^{24}\)MgH\(^+\) by a broadband laser source (\(\Gamma_L/2\pi \gtrsim\) 50 MHz). The atomic ion \(^{24}\)Mg\(^+\) has been picked to introduce a simple system to make comparisons with experimental results while still capturing most of the physics involved in electronic excitations of molecular ions.
Phys. Rev. Lett. 125, 123003 (2020) We perform high-resolution spectroscopy of the $3$d$~^2$D$_{3/2} -
3$d$~^2$D$_{5/2}$ interval in all stable even isotopes of $^A$Ca$^+$ (A = 40,
42, 44, 46 and 48) ...with an accuracy of $\sim$ 20 Hz using direct frequency-comb
Raman spectroscopy. Combining these data with isotope shift measurements of the
4s$~^2$S$_{1/2} \leftrightarrow 3$d$~^2$D$_{5/2}$ transition, we carry out a
King plot analysis with unprecedented sensitivity to coupling between electrons
and neutrons by bosons beyond the Standard Model. Furthermore, we estimate the
sensitivity to such bosons from equivalent spectroscopy in Ba$^+$ and Yb$^+$.
Finally, the data yield isotope shifts of the 4s$~^2$S$_{1/2} \leftrightarrow
3$d$~^2$D$_{3/2}$ transition at 10 part-per-billion through combination with
recent data of Knollmann et al (2019).
We report on the use of an ultracold ensemble of $^{87}$Rb atoms trapped in a
vertical lattice as a source for a quantum force sensor based on a Ramsey-Raman
type interferometer. We reach spatial ...resolution in the low micrometer range in
the vertical direction thanks to evaporative cooling down to ultracold
temperatures in a crossed optical dipole trap. In this configuration, the
coherence time of the atomic ensemble is degraded by inhomogeneous dephasing
arising from atomic interactions. By weakening the confinement in the
transverse direction only, we dilute the cloud and drastically reduce the
strength of these interactions, without affecting the vertical resolution. This
allows to maintain an excellent relative sensitivity on the Bloch frequency,
which is related to the local gravitational force, of $5\times10^{-6}$ at 1\,s
which integrates down to $8\times10^{-8}$ after one hour averaging time.
We investigate the collective spin dynamics of a self-rephasing bosonic ensemble of \(^{87}\)Rb trapped in a 1D vertical optical lattice. We show that the combination of the frequency shifts induced ...by atomic interactions and inhomogeneous dephasing, together with the spin self-rephasing mechanism leads to the existence of a `magic density': \textit{i.e} a singular operating point where the clock transition is first-order insensitive to density fluctuations. This feature is very appealing for improving the stability of quantum sensors based on trapped pseudo-spin-1/2 ensembles. Ramsey spectroscopy of the \(|F=1,m_{F}=0\rangle\rightarrow|F=2,m_{F}=0\rangle\) hyperfine transition is in qualitative agreement with a numerical model based on coupled Bloch equations of motion for energy dependent spin vectors.
We demonstrate the use of a femtosecond frequency comb to coherently drive stimulated Raman transitions between terahertz-spaced atomic energy levels. More specifically, we address the ...\(3d~^2D_{3/2}\) and \(3d~^2D_{5/2}\) fine structure levels of a single trapped \(^{40}\)Ca\(^+\) ion and spectroscopically resolve the transition frequency to be \(\nu_D = 1{,}819{,}599{,}021{,}534 \pm 8\) Hz. The achieved accuracy is nearly a factor of five better than the previous best Raman spectroscopy, and is currently limited by the stability of our atomic clock reference. Furthermore, the population dynamics of frequency-comb-driven Raman transitions can be fully predicted from the spectral properties of the frequency comb, and Rabi oscillations with a contrast of 99.3(6)\% and millisecond coherence time has been achieved. Importantly, the technique can be easily generalized to transitions in the sub-kHz to tens of THz range and should be applicable for driving, e.g., spin-resolved rovibrational transitions in molecules and hyperfine transitions in highly charged ions.
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