We studied the properties of a cold (ϳ100 mK) and dense (ϳ10 8 10 cm 23) atomic Rydberg Cs gas, and found that the observed widths and shapes of resonances in population transfers cannot be explained in the framework of a usual gas model. We propose a "frozen Rydberg gas" model, where the interplay between two-body and many-body phenomena affects in an unexpected way the width and the shape of spectral lines. [S0031-9007(97)04903-X]

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We develop a quantum-mechanical approach to the photoassociation phenomenon in a sample of cold atoms initially at thermal equilibrium with temperature T . We calculate the rate of formation of molecules in the weak field regime using a perturbative approach. Analytical expressions are obtained for the overlap between the initial and final molecular wavefunctions, and are shown to agree with numerical calculations. The photoassociation rate is found to decrease with increasing atomic mass m, from Li to Cs. Also considered is the heating of the atomic sample through spontaneous photodissociation. † This index n is simply used to number the rovibrational molecular states and has nothing to do with the principal quantum number which characterizes the atomic ns and np states.

We propose and experimentally demonstrate a method to prepare a nonspreading atomic wave packet. Our technique relies on a spatially modulated absorption constantly chiseling away from an initially broad de Broglie wave. The resulting contraction is balanced by dispersion due to Heisenberg's uncertainty principle. This quantum evolution results in the formation of a nonspreading wave packet of Gaussian form with a spatially quadratic phase. Experimentally, we confirm these predictions by observing the evolution of the momentum distribution. Moreover, by employing interferometric techniques, we measure the predicted quadratic phase across the wave packet. Nonspreading wave packets of this kind also exist in two space dimensions and we can control their amplitude and phase using optical elements.PACS numbers: 03.75. Be, 42.50.Vk, 03.75.Dg Nonspreading wave packets have attracted interest since the early days of quantum mechanics. Already in 1926 Schrödinger [1] found that the displaced Gaussian ground state of a harmonic oscillator experiences conformal evolution because a classical force prevents the wave packet from spreading. Even in free space the correlations between position and momentum stored in an initially Airy-function-shaped wave packet can prevent spreading [2]. Here we propose and experimentally observe the formation and propagation of nondispersive atomic wave packets in an imaginary (absorptive) potential accessible in atom optics [3,4,5]. Although there is no classical force, there are correlations continuously imposed by Heisenberg's uncertainty relation resulting in the stabilization of the wave packet.Localized wave packets due to stabilization are well known in the context of periodically driven quantum systems [6] and studied with increasing interest for electronic wave packets in Rydberg atoms [7,8,9,10]. Our approach to create nondispersive atomic wave packets relies on three ingredients: (i) an absorption process [11] cuts away the unwanted parts of a broad wave creating a packet that is continuously contracting in position space, (ii) this process leads due to Heisenberg's uncertainty relation to a broadening in momentum space and consequently to a faster spreading in real space, and (iii) the absorptive narrowing and the quantum spreading are balanced, leading to a nonspreading wave packet. In the following we will refer to such a wave packet as Michelangelo packet [12].Complex potentials for matter waves [13] emerge from the interaction of near resonant light with an open twolevel system shown in Fig. 1(a). For a standing light wave tuned exactly on resonance an array of purely imaginary harmonic potentials arises. When the Rabi frequency Ω 0 is of the order of the excited state linewidth Γ the local saturation parameter |Ω 0 sin(kx)/Γ|, and thus the upper level population, is of the order of unity except in a small vicinity of the field nodes. Consequently, our system decays approximately with the rate Γ. Therefore, in the time domain t ≫ 1/Γ the atomic wave function vanishes almost...

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