Probing the light induced dipole-dipole interaction in momentum space

We theoretically investigate the mechanical effect of the light-induced dipole-dipole interaction potential on the atoms in a Bose-Einstein condensate. We present numerical calculations on the magnitude and shape of the induced potentials for different experimentally accessible geometries. It is shown that the mechanical effect can be distinguished from the effect of incoherent scattering for an experimentally feasible setting

Measurement error and rank correlations

This paper characterizes and proposes a method to correct for errors-in-variables biases in the estimation of rank correlation coefficients (Spearman’s ρ and Kendall’s τ ). We first investigate a set of sufficient conditions under which measurement errors bias the sample rank correlations toward zero. We then provide a feasible nonparametric bias-corrected estimator based on the technique of small error variance approximation. We assess its performance in simulations and an empirical application, using rich Swedish data to estimate intergenerational rank correlations in income. The method performs well in both cases, lowering the mean squared error by 50-85 percent already in moderately sized samples (n = 1, 000)

A two species trap for chromium and rubidium atoms

We realize a combined trap for bosonic chromium 52Cr and rubidium 87Rb atoms. First experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross section of sigma_{inelRbCr}=(5.0+-4.0)*10^{-18}m^2 for light induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross section of the excited 5P_{3/2} state at an ionizing wavelength of 426nm is measured to be sigma_{p}=(1.1+-0.3)*10^{-21}m^2

Essays on Migration and Intergenerational Mobility

My dissertation addresses questions in two topic areas, intergenerational mobility and migration. I first study the dynamic response of income mobility to structural changes in a model of intergenerational transmission. I illustrate that mobility today depends on past policies and institutions, such that a decline in mobility may reflect past gains rather than a recent deterioration of “equality of opportunity”. How to measure income mobility is addressed in the next chapter, in which I document that heterogeneity in the shape of income profiles generates large life-cycle biases, which cannot be eliminated with standard methods used in the current literature. Finally, in the fourth chapter I study how elements of the transmission process affect the relation between mobility over two generations and the long-run persistence of economic status within families. I provide various arguments why long-run mobility is likely lower than predictions from intergenerational evidence suggest. In the final chapter I analyse the effect of migration on labor markets. Triggered by the fall of the iron curtain, Germany experienced a sudden inflow of Czech workers that reached a local employment share of up to ten percent. I exploit this natural experiment to assess how immigration affects native workers, and to examine the mechanisms by which labor markets adjust. I find a strong and rapid response in both native wages and employment, and document substantial heterogeneity across age groups: native employment decreases most strongly among older workers, even though their wages are less affected than for other age groups. This finding suggests that the elasticity of labor supply differs across demographic groups, with important implications for the analysis of responses to labor supply shocks. When distinguishing between the different types of adjustment, I find that native employment decreases predominantly through diminished inflows into work, less so through outflows into non-employment

Expansion dynamics of a dipolar Bose-Einstein condensate

Our recent measurements on the expansion of a chromium dipolar condensate after release from an optical trapping potential are in good agreement with an exact solution of the hydrodynamic equations for dipolar Bose gases. We report here the theoretical method used to interpret the measurement data as well as more details of the experiment and its analysis. The theory reported here is a tool for the investigation of different dynamical situations in time-dependent harmonic traps.Comment: 12 pages. Submitted to PR

Production of a chromium Bose-Einstein condensate

The recent achievement of Bose-Einstein condensation of chromium atoms [1] has opened longed-for experimental access to a degenerate quantum gas with long-range and anisotropic interaction. Due to the large magnetic moment of chromium atoms of 6 {$\mu$}B, in contrast to other Bose- Einstein condensates (BECs), magnetic dipole-dipole interaction plays an important role in a chromium BEC. Many new physical properties of degenerate gases arising from these magnetic forces have been predicted in the past and can now be studied experimentally. Besides these phenomena, the large dipole moment leads to a breakdown of standard methods for the creation of a chromium BEC. Cooling and trapping methods had to be adapted to the special electronic structure of chromium to reach the regime of quantum degeneracy. Some of them apply generally to gases with large dipolar forces. We present here a detailed discussion of the experimental techniques which are used to create a chromium BEC and alow us to produce pure condensates with up to {$10^5$} atoms in an optical dipole trap. We also describe the methods used to determine the trapping parameters.Comment: 17 pages, 9 figure

Depolarisation cooling of an atomic cloud

We propose a cooling scheme based on depolarisation of a polarised cloud of trapped atoms. Similar to adiabatic demagnetisation, we suggest to use the coupling between the internal spin reservoir of the cloud and the external kinetic reservoir via dipolar relaxation to reduce the temperature of the cloud. By optical pumping one can cool the spin reservoir and force the cooling process. In case of a trapped gas of dipolar chromium atoms, we show that this cooling technique can be performed continuously and used to approach the critical phase space density for BECComment: 8 pages, 5 figure

Observation of Feshbach resonances in an ultracold gas of 52{}^{52}Cr

We have observed Feshbach resonances in elastic collisions between ultracold ${}^{52}$Cr atoms. This is the first observation of collisional Feshbach resonances in an atomic species with more than one valence electron. The zero nuclear spin of ${}^{52}$Cr and thus the absence of a Fermi-contact interaction leads to regularly-spaced resonance sequences. By comparing resonance positions with multi-channel scattering calculations we determine the s-wave scattering length of the lowest $^{2S+1}\Sigma_{g}^{+}$ potentials to be \unit[112(14)]{a_0}, \unit[58(6)]{a_0} and -\unit[7(20)]{a_0} for S=6, 4, and 2, respectively, where a_{0}=\unit[0.0529]{nm}.Comment: 4 pages, 2 figures, 1 tabl

Atom interferometry gravity-gradiometer for the determination of the Newtonian gravitational constant G

We developed a gravity-gradiometer based on atom interferometry for the determination of the Newtonian gravitational constant \textit{G}. The apparatus, combining a Rb fountain, Raman interferometry and a juggling scheme for fast launch of two atomic clouds, was specifically designed to reduce possible systematic effects. We present instrument performances and show that the sensor is able to detect the gravitational field induced by source masses. A discussion of projected accuracy for \textit{G} measurement using this new scheme shows that the results of the experiment will be significant to discriminate between previous inconsistent values.Comment: 9 pages,9 figures, Submitte