Quantum Repeaters based on Single Trapped Ions

We analyze the performance of a quantum repeater protocol based on single trapped ions. At each node, single trapped ions embedded into high finesse cavities emit single photons whose polarization is entangled with the ion state. A specific detection of two photons at a central station located half-way between two nodes heralds the entanglement of two remote ions. Entanglement can be extended to long distances by applying successive entanglement swapping operations based on two-ion gate operations that have already been demonstrated experimentally with high precision. Our calculation shows that the distribution rate of entanglement achievable with such an ion-based quantum repeater protocol is higher by orders of magnitude than the rates that are achievable with the best known schemes based on atomic ensemble memories and linear optics. The main reason is that for trapped ions the entanglement swapping operations are performed deterministically, in contrast to success probabilities below 50 percent per swapping with linear optics. The scheme requires efficient collection of the emitted photons, which can be achieved with cavities, and efficient conversion of their wavelength, which can be done via stimulated parametric down-conversion. We also suggest how to realize temporal multiplexing, which offers additional significant speed-ups in entanglement distribution, with trapped ions

Electric field noise above surfaces: a model for heating rate scaling law in ion traps

We present a model for the scaling laws of the electric field noise spectral density as a function of the distance, $d$, above a conducting surface. Our analytical approach models the patch potentials by introducing a correlation length, $\zeta$, of the electric potential on the surface. The predicted scaling laws are in excellent agreement with two different classes of experiments (cold trapped ions and cantilevers), that span at least four orders of magnitude of $d$. According to this model, heating rate in miniature ion traps could be greatly reduced by proper material engineering

Breakdown of scale invariance in a quasi-two-dimensional Bose gas due to the presence of the third dimension

In this Rapid Communication, we describe how the presence of the third dimension may break the scale invariance in a two-dimensional Bose gas in a pancake-shaped trap. From the two-dimensional perspective, the possibility of a weak spilling of the atomic density beyond the ground-state of the confinement alters the two-dimensional chemical potential; in turn, this correction no longer supports scale invariance. We compare experimental data with numerical and analytic perturbative results and find a good agreement.Comment: 4 pages, 1 figure, published in PRA Rapid Com

Imaging the collective excitations of an ultracold gas using statistical correlations

Advanced data analysis techniques have proved to be crucial for extracting information from noisy images. Here we show that principal component analysis can be successfully applied to ultracold gases to unveil their collective excitations. By analyzing the correlations in a series of images we are able to identify the collective modes which are excited, determine their population, image their eigenfunction, and measure their frequency. Our method allows to discriminate the relevant modes from other noise components and is robust with respect to the data sampling procedure. It can be extended to other dynamical systems including cavity polariton quantum gases or trapped ions.Comment: See also the supplementary material and the video abstrac

Isotope shifts of natural Sr+ measured by laser fluorescence in a sympathetically cooled Coulomb crystal

We measured by laser spectroscopy the isotope shifts between naturally-occurring even-isotopes of strontium ions for both the 5s\,\,^2S_{1/2}\to 5p\,\,^2P_{1/2} (violet) and the 4d\,\,^2D_{3/2}\to 5p\,\,^2P_{1/2} (infrared) dipole-allowed optical transitions. Fluorescence spectra were taken by simultaneous measurements on a two-component Coulomb crystal in a linear Paul trap containing $10^3$--$10^4$ laser-cooled Sr$^+$ ions. The isotope shifts are extracted from the experimental spectra by fitting the data with the analytical solution of the optical Bloch equations describing a three-level atom in interaction with two laser beams. This technique allowed us to increase the precision with respect to previously reported data obtained by optogalvanic spectroscopy or fast atomic-beam techniques. The results for the 5s\,\,^2S_{1/2}\to 5p\,\,^2P_{1/2} transition are $\nu_{88}-\nu_{84}=+378(4)$ MHz and $\nu_{88}-\nu_{86}=+170(3)$ MHz, in agreement with previously reported measurements. In the case of the previously unexplored 4d\,\,^2D_{3/2}\to 5p\,\,^2P_{1/2} transition we find $\nu_{88}-\nu_{84}=-828(4)$ MHz and $\nu_{88}-\nu_{86}=-402(2)$ MHz. These results provide more data for stringent tests of theoretical calculations of the isotope shifts of alkali-metal-like atoms. Moreover, they simplify the identification and the addressing of Sr$^+$ isotopes for ion frequency standards or quantum-information-processing applications in the case of multi-isotope ion strings.Comment: 19 pages; 5 figures; accepted on Phys. Rev. A (http://pra.aps.org/

Robust preparation and manipulation of protected qubits using time--varying Hamiltonians

We show that it is possible to initialize and manipulate in a deterministic manner protected qubits using time varying Hamiltonians. Taking advantage of the symmetries of the system, we predict the effect of the noise during the initialization and manipulation. These predictions are in good agreement with numerical simulations. Our study shows that the topological protection remains efficient under realistic experimental conditions.Comment: To be published in Phys. Rev. Let

Probing superfluidity in a quasi two-dimensional Bose gas through its local dynamics

We report direct evidence of superfluidity in a quasi two-dimensional Bose gas by observing its dynamical response to a collective excitation. Relying on a novel local correlation analysis, we are able to probe inhomogeneous clouds and reveal their local dynamics. We identify in this way the superfluid and thermal phases inside the gas and locate the boundary at which the Berezinskii--Kosterlitz--Thouless crossover occurs. This new analysis also allows to evidence the coupling of the two fluids which induces at finite temperatures damping rates larger than the usual Landau damping

Influence of the Radio-Frequency source properties on RF-based atom traps

We discuss the quality required for the RF source used to trap neutral atoms in RF-dressed potentials. We illustrate this discussion with experimental results obtained on a Bose-Einstein condensation experiment with different RF sources.Comment: 5 figures, 7 page

DMTs and Covid-19 severity in MS: a pooled analysis from Italy and France

We evaluated the effect of DMTs on Covid-19 severity in patients with MS, with a pooled-analysis of two large cohorts from Italy and France. The association of baseline characteristics and DMTs with Covid-19 severity was assessed by multivariate ordinal-logistic models and pooled by a fixed-effect meta-analysis. 1066 patients with MS from Italy and 721 from France were included. In the multivariate model, anti-CD20 therapies were significantly associated (OR = 2.05, 95%CI = 1.39–3.02, p < 0.001) with Covid-19 severity, whereas interferon indicated a decreased risk (OR = 0.42, 95%CI = 0.18–0.99, p = 0.047). This pooled-analysis confirms an increased risk of severe Covid-19 in patients on anti-CD20 therapies and supports the protective role of interferon

Réalisation, étude et exploitation d'ensembles d'ions refroidis par laser stockés dans des pièges micro-fabriqués pour l'information quantique

This PHD work aims at developing a micro-fabricated ion trap for quantum information and computation purposes. Laser cooled trapped ions are indeed among the most promising systems to study and develop quantum information protocols: they have long coherence time and their quantum state may be entirely controlled. However ion trapping experiments are not easily scalable and trap miniaturization is an experimental challenge. This work contains an introduction to quantum information experiments with trapped ions and study the original case of quantum repeaters based on single trapped ions. In a second part, trap miniaturization is discussed emphasizing the optimization of the trap geometry. An analytical model is introduced to take into account surface charge induced fluctuating electric field in the planar ion trap geometry. The scaling of these fluctuations with the ion to electrode surface distance is discussed, as well as a possible optimization protocol based on atomic force microscopy measurements. Finally the experimental developments and results are presented, including clean room trap micro-fabrication optimization, single ion trapping and cooling in a linear Paul trap and preliminary heating rates measurements.Cette thèse s'inscrit dans le domaine général de l'information quantique, qui cherche à tirer parti des lois de la mécanique quantique pour proposer des protocoles de traitement de l'information originaux. Ce travail se focalise sur l'un des systèmes les plus prometteurs en terme d'information quantique : les ions uniques confinés dans des pièges radio-fréquence et refroidis par laser. Une étude théorique a permis de proposer un protocole original d'échange d'état intriqués sur de longues distances, utilisant des ions piégés. L'étude de la miniaturisation des dispositifs de piégeage, permettant d'envisager la construction de futurs ordinateurs quantiques, passe par la compréhension de l'effet des défauts des électrodes sur un ion piégé : cette thèse propose un modèle analytique original permettant de calculer la loi d'échelle de l'effet des bruits électriques avec la taille du piège. Ce modèle servira à optimiser le protocole de fabrication de pièges miniatures, dont les premiers échantillons ont été réalisés et sont en cours de test sur un dispositif expérimental dédié (enceinte à vide, électronique de contrôle et sources laser de refroidissement). Les premiers résultats prometteurs ont permis de démontrer le piégeage et le refroidissement d'ion uniques de Strontium refroidis par laser, dans un piège de Paul linéaire macroscopique