Laser Cooling of Molecular Anions

We propose a scheme for laser cooling of negatively charged molecules. We briefly summarise the requirements for such laser cooling and we identify a number of potential candidates. A detailed computation study with C$\_2^-$, the most studied molecular anion, is carried out. Simulations of 3D laser cooling in a gas phase show that this molecule could be cooled down to below 1 mK in only a few tens of milliseconds, using standard lasers. Sisyphus cooling, where no photo-detachment process is present, as well as Doppler laser cooling of trapped C$\_2^-$, are also simulated. This cooling scheme has an impact on the study of cold molecules, molecular anions, charged particle sources and antimatter physics

1S-3S cw spectroscopy of hydrogen/deuterium atom

We study the 1S-3S two-photon transition of hydrogen in a thermal atomic beam, using a homemade cw laser source at 205 nm. The experimental method is described, leading in 2017 to the measurement of the 1S-3S transition frequency in hydrogen atom with a relative uncertainty of $9 \times 10^{-13}$. This result contributes to the "proton puzzle" resolution but is in disagreement with the ones of some others experiments. We have recently improved our setup with the aim of carrying out the same measurement in deuterium. With the improved detection system, we have observed a broadened fluorescence signal, superimposed on the narrow signal studied so far, and due to the stray accumulation of atoms in the vacuum chamber. The possible resulting systematic effect is discussed

Experimental perspectives on the matter-antimatter asymmetry puzzle: developments in electron EDM and antihydrogen experiments

In the search for clues to the matter-antimatter puzzle, experiments with atoms or molecules play a particular role. These systems allow measurements with very high precision, as demonstrated by the unprecedented limits down to $10^{-30}$ e.cm on electron EDM using molecular ions, and relative measurements at the level of $10^{-12}$ in spectroscopy of antihydrogen atoms. Building on these impressive measurements, new experimental directions offer potentials for drastic improvements. We review here some of the new perspectives in those fields and their associated prospects for new physics searches

A compact 20-pass thin-disk multipass amplifier stable against thermal lensing effects and delivering 330 mJ pulses with M2<1.17\bf{M^2 < 1.17}

We report on an Yb:YAG thin-disk multipass amplifier delivering 50 ns long pulses at a central wavelength of 1030 nm with an energy of 330 mJ at a repetition rate of 100 Hz. The beam quality factor at the maximum energy was measured to be $\text{M}^2 = 1.17$. The small signal gain is 20, and the gain at 330 mJ was measured to be 6.9. The 20-pass amplifier is designed as a concatenation of stable resonator segments in which the beam is alternately Fourier transformed and relay-imaged back to the disk by a 4f-imaging optical scheme stage. The Fourier transform propagation makes the output beam robust against spherical phase front distortions, while the 4f-stage is used to compensate the thermal lens of the thin-disk and to reduce the footprint of the amplifier

Manipulation et refroidissement laser de l'antimatière, au sein de l'expérience AEgIS

My Ph.D project took place within the AEgIS collaboration, one of the antimatter experiments at the CERN. The final goal of the experiment is to perform a gravity test on a cold antihydrogen (Hbar) beam. AEgIS proposes to create such a cold Hbar beam based on a charge exchange reaction between excited Rydberg Positronium (Ps) and cold trapped antiprotons: 〖Ps〗^* + pbar → (H^*)⁻ + e⁻. Studying the Ps physics is crucial for the experiment, and requires adapted lasers systems. During this Ph.D, my primary undertaking was the responsibility for the laser systems in AEgIS. To excite Ps atom up to its Rydberg states (≃20) in presence of a high magnetic field (1 T), two broadband pulsed lasers have been developed. We realized the first laser excitation of the Ps into the n=3 level, and demonstrated an efficient optical path to reach the Rydberg state n=16-17. These results, obtained in the vacuum test chamber and in absence of strong magnetic field, reach a milestone toward the formation of antihydrogen in AEgIS, and the immediate next step for us is to excite Ps atoms inside our 1 T trapping apparatus, where the formation of antihydrogen will take place. However, even once this next step will be successful, the production rate of antihydrogen atoms will nevertheless be very low, and their temperature much higher than could be wished. During my Ph.D, I have installed further excitation lasers, foreseen to perform fine spectroscopy on Ps atoms and that excite optical transitions suitable for a possible Doppler cooling. I have carried out theoretical studies and simulations to determine the proper characteristics required for a cooling laser system. The transverse laser cooling of the Ps beam will enhance the overlap between the trapped antiprotons plasma and the Ps beam during the charge-exchange process, and therefore drastically improve the production rate of antihydrogen. The control of the compression and cooling of the antiproton plasma is also crucial for the antihydrogen formation. During the beam-times of 2014 and 2015, I participated in the characterization and optimization our catching and manipulation procedures to reach highly compressed antiproton plasma, in repeatable conditions. Another project in AEgIS I took part aims to improve the formation rate of ultracold antihydrogen, by studying the possibility of a sympathetically cooling of the antiprotons using a laser-cooled anion plasma. I investigated some laser cooling schemes on the C₂⁻ molecular anions, and the simulations are promising. I actively contribute to the commissioning of the test apparatus at CERN to carry on the trials of laser cooling on the C₂⁻ species. If successful, this result will not only be the first cooling of anions by laser, but will open the way to a highly efficient production of ultracold antihydrogen atoms.Ma thèse s’est déroulée dans le cadre de la collaboration AEgIS, une des expériences étudiant l’antimatière au CERN. L’objectif final est de mesurer l’effet de la gravité sur un faisceau froid d’antihydrogène (Hbar). AEgIS se propose de créer les Hbar froids par échange de charges entre un atome de Positronium (Ps) excité (état de Rydberg) et un antiproton piégé : 〖Ps〗^*+ pbar → (H^*)⁻ + e⁻. L’étude de la physique du Ps est cruciale pour AEgIS, et demande des systèmes lasers adaptés. Pendant ma thèse, ma première tâche a été de veiller au bon fonctionnement des systèmes lasers de l’expérience. Afin d’exciter le positronium jusqu’à ses états de Rydberg (≃20) en présence d’un fort champ magnétique (1 T), deux lasers pulsés spectralement larges ont été spécialement conçu. Nous avons réalisé la première excitation par laser du Ps dans son niveau n=3, et prouvé une excitation efficace du nuage de Ps vers les niveaux de Rydberg n=16-17. Ces mesures, réalisées dans la chambre à vide de test d’AEgIS, à température ambiance et pour un faible champ magnétique environnant, sont la première étape vers la formation d’antihydrogène. Le prochain objectif est de répéter ces résultats dans l’enceinte du piège à 1 T, où les antihydrogènes seront formés. Pour autant, malgré l’excitation Rydberg des Ps pour accroître la section efficace de collision, la production d’antihydrogène restera faible, et la température des H bar formés sera trop élevée pour toute mesure de gravité. Pendant ma thèse, j’ai installé au CERN un autre système laser prévu pour pratiquer une spectroscopie précise des niveaux de Rydberg du Ps. Ce système excite des transitions optiques qui pourraient convenir à un refroidissement Doppler : la transition n=1 ↔ n=2. J’ai étudié la possibilité d’un tel refroidissement, en procédant à des simulations poussées pour déterminer les caractéristiques d’un système laser adapté La focalisation du nuage de Ps grâce au refroidissement des vitesses transverses devrait accroitre le recouvrement des positroniums avec les antiprotons piégés, et ainsi augmenter grandement la production d’Hbar. Le contrôle du refroidissement et de la compression du plasma d’antiprotons est aussi essentiel pour la formation des antihydrogènes. Pendant les temps de faisceaux d’antiprotons de 2014 et 2015, j’ai contribué à la caractérisation et l’optimisation des procédures pour attraper et manipuler les antiprotons, afin d’atteindre des plasmas très denses, et ce, de façon reproductible. Enfin, j’ai participé activement à l’élaboration d’autre projet à l’étude AEgIS, qui vise aussi à augmenter la production d’antihydrogène : le projet d’un refroidissement sympathique des antiprotons, en utilisant un plasma d’anions refroidis par laser. J’ai étudié la possibilité de refroidir l’ion moléculaire C₂⁻, et les résultats de simulations sont encourageants. Nous sommes actuellement en train de développer au CERN le système expérimental qui nous permettra de faire les premiers tests de refroidissement sur le C₂⁻. Si couronné de succès, ce projet ne sera pas seulement le premier résultat de refroidissement par laser d’anions, mais ouvrira aussi les portes à une production efficace d’antihydrogènes froids

Positronium laser cooling in a magnetic field

We study realistic 3D laser cooling of positronium (Ps) in the presence of a magnetic field. Triplet and singlet states mixing due to the magnetic field, and dynamical Stark effect, generally produce higher annihilation rates than in the zero-field case. 3D cooling is efficient only at very low field B≲50mT and at high field values B⪆0.7T. Near 100ns long laser pulses, spectrally broad enough to cover most of the Ps Doppler profile and with energy in the mJ range, are required to cool Ps. Simulations based on full diagonalization of the Stark and Zeeman Hamiltonian and a kinetic Monte Carlo algorithm exactly solving the rate equations indicate that an efficient cooling (typically from 300K down to below 50K) is possible even in a magnetic field. We also propose 3D moving molasses cooling that can produce a well-defined monochromatic Ps beam useful for applications

1S–3S cw spectroscopy of hydrogen/deuterium atom

International audienceWe study the 1S-3S two-photon transition of hydrogen in a thermal atomic beam, using a homemade cw laser source at 205 nm. The experimental method is described, leading in 2017 to the measurement of the 1S-3S transition frequency in hydrogen atom with a relative uncertainty of 9 × 10 −13. This result contributes to the "proton puzzle" resolution but is in disagreement with the ones of some others experiments. We have recently improved our setup with the aim of carrying out the same measurement in deuterium. With the improved detection system, we have observed a broadened fluorescence signal, superimposed on the narrow signal studied so far, and due to the stray accumulation of atoms in the vacuum chamber. The possible resulting systematic effect is discussed

GRASIAN: towards the first demonstration of gravitational quantum states of atoms with a cryogenic hydrogen beam

At very low energies, a light neutral particle above a horizontal surface can experience quantum reflection. The quantum reflection holds the particle against gravity and leads to gravitational quantum states (GQs). So far, GQs were only observed with neutrons as pioneered by Nesvizhevsky and his collaborators at ILL. However, the existence of GQs is predicted also for atoms. The GRAsIAN collaboration pursues the first observation and studies of GQs of atomic hydrogen. We propose to use atoms in order to exploit the fact that orders of magnitude larger fluxes compared to those of neutrons are available. Moreover, recently the q -BOUNCE collaboration, performing GQs spectroscopy with neutrons, reported a discrepancy between theoretical calculations and experiment which deserves further investigations. For this purpose, we set up a cryogenic hydrogen beam at 6 K. We report on our preliminary results, characterizing the hydrogen beam with pulsed laser ionization diagnostics at 243 nm.ISSN:1434-6060ISSN:1434-607

Pound–Drever–Hall locking scheme free from Trojan operating points

The Pound–Drever–Hall (PDH) technique is a popular method for stabilizing the frequency of a laser to a stable optical resonator or, vice versa, the length of a resonator to the frequency of a stable laser. We propose a refinement of the technique yielding an “infinite” dynamic (capture) range so that a resonator is correctly locked to the seed frequency, even after large perturbations. The stable but off-resonant lock points (also called Trojan operating points), present in conventional PDH error signals, are removed by phase modulating the seed laser at a frequency corresponding to half the free spectral range of the resonator. We verify the robustness of our scheme experimentally by realizing an injection-seeded Yb:YAG thin-disk laser. We also give an analytical formulation of the PDH error signal for arbitrary modulation frequencies and discuss the parameter range for which our PDH locking scheme guarantees correct locking. Our scheme is simple as it does not require additional electronics apart from the standard PDH setup and is particularly suited to realize injection-seeded lasers and injection-seeded optical parametric oscillators.ISSN:0034-6748ISSN:1089-762