Stabilization of Ultracold Molecules Using Optimal Control Theory

In recent experiments on ultracold matter, molecules have been produced from ultracold atoms by photoassociation, Feshbach resonances, and three-body recombination. The created molecules are translationally cold, but vibrationally highly excited. This will eventually lead them to be lost from the trap due to collisions. We propose shaped laser pulses to transfer these highly excited molecules to their ground vibrational level. Optimal control theory is employed to find the light field that will carry out this task with minimum intensity. We present results for the sodium dimer. The final target can be reached to within 99% if the initial guess field is physically motivated. We find that the optimal fields contain the transition frequencies required by a good Franck-Condon pumping scheme. The analysis is able to identify the ranges of intensity and pulse duration which are able to achieve this task before other competing process take place. Such a scheme could produce stable ultracold molecular samples or even stable molecular Bose-Einstein condensates

Ultracold heteronuclear molecules in a 3D optical lattice

We report on the creation of ultracold heteronuclear molecules assembled from fermionic 40K and bosonic 87Rb atoms in a 3D optical lattice. Molecules are produced at a heteronuclear Feshbach resonance both on the attractive and the repulsive side of the resonance. We precisely determine the binding energy of the heteronuclear molecules from rf spectroscopy across the Feshbach resonance. We characterize the lifetime of the molecular sample as a function of magnetic field and measure between 20 and 120ms. The efficiency of molecule creation via rf association is measured and is found to decrease as expected for more deeply bound molecules.Comment: 4 pages, 4 figure

Photoassociation of cold atoms with chirped laser pulses: time-dependent calculations and analysis of the adiabatic transfer within a two-state model

This theoretical paper presents numerical calculations for photoassociation of ultracold cesium atoms with a chirped laser pulse and detailed analysis of the results. In contrast with earlier work, the initial state is represented by a stationary continuum wavefunction. In the chosen example, it is shown that an important population transfer is achieved to $\approx 15$ vibrational levels in the vicinity of the v=98 bound level in the external well of the $0_g^-(6s+6p_{3/2})$ potential. Such levels lie in the energy range swept by the instantaneous frequency of the pulse, thus defining a ``photoassociation window''. Levels outside this window may be significantly excited during the pulse, but no population remains there after the pulse. Finally, the population transfer to the last vibrational levels of the ground $a^3\Sigma_u^+$(6s + 6s) is significant, making stable molecules. The results are interpreted in the framework of a two state model as an adiabatic inversion mechanism, efficient only within the photoassociation window. The large value found for the photoassociation rate suggests promising applications. The present chirp has been designed in view of creating a vibrational wavepacket in the excited state which is focussing at the barrier of the double well potential.Comment: 49 pages, 9 figures, submitted to Phys. Rev.

Recovering missing data on satellite images

International audienceData Assimilation is commonly used in environmental sciences to improve forecasts, obtained by meteorological, oceanographic or air quality simulation models, with observation data. It aims to solve an evolution equation, describing the dynamics, and an observation equation, measuring the misfit between the state vector and the observations, to get a better knowledge of the actual system's state, named the reference. In this article, we describe how to use this technique to recover missing data and reduce noise on satellite images. The recovering process is based on assumptions on the underlying dynamics displayed by the sequence of images. This is a promising alternative to methods such as space-time interpolation. In order to better evaluate our approach, results are first quantified for an artificial noise applied on the acquisitions and then displayed for real data

Differences Between The Optical/Uv Spectra Of X-Ray Bright And X-Ray Faint QSOs

We contrast measurements of composite optical and ultraviolet (UV) spectra constructed from samples of QSOs defined by their soft X-ray brightness. X-ray bright (XB) composites show stronger emission lines in general, but particularly from the narrow line region. The difference in the [OIII]/Hbeta ratio is particularly striking, and even more so when blended FeII emission is properly subtracted. The correlation of this ratio with X-ray brightness were principal components of QSO spectral diversity found by Boroson & Green (1992). We find here that other, much weaker narrow optical forbidden lines ([OII] and NeV) are enhanced by factors of 2 to 3 in our XB composites, and that narrow line emission is also strongly enhanced in the XB UV composite. Broad permitted line fluxes are slightly larger for all XB spectra, but the narrow/broad line ratio stays similar or increases strongly with X-ray brightness for all strong permitted lines except Hbeta. Spectral differences between samples divided by their relative X-ray brightness (as measured by alpha_{ox}) exceed those seen between complementary samples divided by luminosity or radio loudness. We propose that the Baldwin effect may be a secondary correlation to the primary relationship between alpha_{ox} and emission line equivalent width. We conclude that either 1) equivalent width depends strongly on the SHAPE of the ionizing continuum, as crudely characterized here by alpha_{ox} or 2) both equivalent width and alpha_{ox} are related to some third parameter characterizing the QSO physics. One such possibility is intrinsic warm absorption; a soft X-ray absorber situated between the broad and narrow line regions can successfully account for many of the properties observed.Comment: 16 pages including 3 figures, AAS latex, plus 4 tables totaling 5 pages, to appear in ApJ Vol. 498, May 1, 199

Frequency shifts of photoassociative spectra of ultracold metastable Helium atoms : a new measurement of the s-wave scattering length

We observe light-induced frequency shifts in one-color photoassociative spectra of magnetically trapped $^4$He$^*$ atoms in the metastable $2^3S_1$ state. A pair of ultracold spin-polarized $2^3S_1$ helium atoms is excited into a molecular bound state in the purely long range $0_u^+$ potential connected to the $2^3S_1 - 2^3P_0$ asymptote. The shift arises from the optical coupling of the molecular excited bound state with the scattering states and the bound states of two colliding $2^3S_1$ atoms. We measure the frequency-shifts for several ro-vibrational levels in the $0^+_u$ potential and find a linear dependence on the photoassociation laser intensity. Comparison with a theoretical analysis provides a good indication for the s-wave scattering length $a$ of the quintet ($^5\Sigma_g^+$) potential, $a=7.2\pm 0.6$ nm, which is significantly lower than most previous results obtained by non-spectroscopic methods.Comment: 7 pages, 4 figure

On the Role of Penning Ionization in Photoassociation Spectroscopy

We study the role of Penning ionization on the photoassociation spectra of He(^3S)-He(^3S). The experimental setup is discussed and experimental results for different intensities of the probe laser are shown. For modelling the experimental results we consider coupled-channel calculations of the crossing of the ground state with the excited state at the Condon point. The coupled-channel calculations are first applied to model systems, where we consider two coupled channels without ionization, two coupled channels with ionization, and three coupled channels, for which only one of the excited states is ionizing. Finally, coupled-channel calculations are applied to photoassociation of He(^3S)-He(^3S) and good agreement is obtained between the model and the experimental results.Comment: 14 pages, 18 figures, submitted to the special issue on Cold Molecules of J. Phys.

Formation of Ultracold Heteronuclear Dimers in Electric Fields

The formation of ultracold molecules via stimulated emission followed by a radiative deexcitation cascade in the presence of a static electric field is investigated. By analyzing the corresponding cross sections, we demonstrate the possibility to populate the lowest rotational excitations via photoassociation. The modification of the radiative cascade due to the electric field leads to narrow rotational state distributions in the vibrational ground state. External fields might therefore represent an additional valuable tool towards the ultimate goal of quantum state preparation of molecules

Photoassociative Production and Trapping of Ultracold KRb Molecules

We have produced ultracold heteronuclear KRb molecules by the process of photoassociation in a two-species magneto-optical trap. Following decay of the photoassociated KRb*, the molecules are detected using two-photon ionization and time-of-flight mass spectroscopy of KRb$^+$. A portion of the metastable triplet molecules thus formed are magnetically trapped. Photoassociative spectra down to 91 cm$^{-1}$ below the K(4$s$) + Rb (5$p_{1/2}$) asymptote have been obtained. We have made assignments to all eight of the attractive Hund's case (c) KRb* potential curves in this spectral region.Comment: 4 pages, 4 figure