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arXiv:0705.3302 · Physical Review A

Modeling many-particle mechanical effects of an interacting Rydberg gas

Thomas Amthor, M. Reetz‐Lamour, Christian Giese, Matthias Weidemüller

Published Nov 9, 2007• 63 citations
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Abstract

In a recent work [Phys. Rev. Lett. 98, 023004 (2007)], we investigated the influence of the attractive van der Waals interaction on the pair distribution and Penning ionization dynamics of ultracold Rydberg gases. Here we extend this description to atoms initially prepared in Rydberg states exhibiting repulsive interaction. We present calculations based on a Monte Carlo algorithm to simulate the dynamics of many atoms under the influence of both repulsive and attractive long-range interatomic forces. Redistribution to nearby states induced by blackbody radiation is taken into account, changing the effective interaction potentials. The model agrees with experimental observations, where the ionization rate is found to increase when the excitation laser is blue detuned from the atomic resonance.

DOI
10.1103/physreva.76.054702

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In a recent work [Phys. Rev. Lett. 98, 023004 (2007)], we investigated the influence of the attractive van der Waals interaction on the pair distribution and Penning ionization dynamics of ultracold Rydberg gases. Here we extend this description to atoms initially prepared in Rydberg states exhibiting repulsive interaction. We present calculations based on a Monte Carlo algorithm to simulate the dynamics of many atoms under the influence of both repulsive and attractive long-range interatomic forces. Redistribution to nearby states induced by blackbody radiation is taken into account, changing the effective interaction potentials. The model agrees with experimental observations, where the ionization rate is found to increase when the excitation laser is blue detuned from the atomic resonance.
We present time-resolved spectroscopic measurements of Rydberg-Rydberg interactions between two Rydberg atoms in an ultracold gas, revealing the pair dynamics induced by long-range van der Waals interactions between the atoms. By detuning the excitation laser, a specific pair distribution is prepared. Penning ionization on a microsecond time scale serves as a probe for the pair dynamics under the influence of the attractive long-range forces. Comparison with a Monte Carlo model not only explains all spectroscopic features but also gives quantitative information about the interaction potentials. The results imply that the interaction-induced ionization rate can be influenced by the excitation laser. Surprisingly, interaction-induced ionization is also observed for Rydberg states with purely repulsive interactions.
Van der Waals interactions between ultracold atoms excited to high-lying Rydberg states can lead to \nstrongly correlated excitation dynamics [1]. In my talk I will present a direct observation in real space \nof the mechanical effect of the van der Waals force on individual Rydberg atoms excited at close range \n[2]. Clusters of Rydberg atoms are created by first generating a small number of seed excitations in a \nmagneto-optical trap, followed by off-resonant excitation that leads to a chain of facilitated excitation \nevents. Our experimental results agree well with a numerical simulation of the van der Waals explosion.
We present the explicit dynamics process of Rydberg cesium atoms initially experiencing a repulsive van der Waals (vdW) interaction by measuring the line width and intensity of the Rydberg ionization spectra. The signals of Rydberg atoms and free ions are recorded simultaneously within an initial 3.5 micros delay time between the excitation laser and the ramp electric field. For high-density gases, a rapid decrease of Rydberg atoms and an increase of free ions are observed, which is not found to be the case for low-density gases. The experimental results indicate that superradiance is the main cause of the redistribution of Rydberg atoms from the repulsive potential to the attractive potential for high density, which provides the initial ionization. The corresponding theoretical calculation is also given.
We investigate the initial ionization process of Rydberg cesium atoms prepared in repulsive van der Waals interaction potential. The time-of-flight spectrum is used to detect the free ions and ionized Rydberg atoms. Free ions are observed immediately after the production of Rydberg atoms and increase significantly in number on the blue detuned side of the resonance of 6P 3/2 →40D 5/2 with increasing delay time between the excitation laser pulse and the ramp ionization field. The initial ionization is explained as the excitation of pair states in attractive potential due to the ac Stark shifts induced by the excitation laser. The attractive dipole–dipole interaction results in fast collision process and penning ionization. We also measure free-ion fraction as a function of delay time. A rapid increase in the number of free ions is observed after a delay time of 1.8 µs. The experimental observations are in accordance with qualitative explanation of the initial ionization mechanism.
Pulsed laser excitation of a dense ultracold Cs vapor has been used to study the pairwise interactions between Cs atoms excited to np3/2n{p}_{3/2}np3/2​ Rydberg states of principal quantum numbers in the range n=22−−36n=22--36n=22−−36. Molecular resonances were observed that correspond to excitation of Rydberg-atom-pair states correlated not only to the np3/2+np3/2n{p}_{3/2}+n{p}_{3/2}np3/2​+np3/2​ dissociation asymptotes, but also to $n{s}_{1/2}+(n+1){s}_{1/2}, n{s}_{1/2}+{n}^{\ensuremath{'}}{f}_{j}$, and $(n\ensuremath{-}4){f}_{j}+(n\ensuremath{-}3){f}_{j}\phantom{\rule{4pt}{0ex}}(j=5/2,7/2)$ dissociation asymptotes. These pair resonances are interpreted as arising from dipole-dipole and higher-order long-range-interaction terms between the Rydberg atoms on the basis of (i) their spectral positions, (ii) their response to static and pulsed electric fields, and (iii) millimeter-wave spectra between pair states correlated to different pair-dissociation asymptotes. The Rydberg-atom-pair states were found to spontaneously decay by Penning ionization and the dynamics of the ionization process were investigated during the first 15 $\ensuremath{\mu}\mathrm{s}$ following initial photoexcitation. To interpret the experimental observations, a potential model was derived that is based on the numerical determination of the eigenvalues and eigenfunctions of the long-range interaction Hamiltonian. With this potential model, which does not include adjustable parameters, all experimental observations could be accounted for, and the results demonstrate that long-range-interaction models provide a global and accurate description of interactions in ultracold Rydberg gases and that they correctly account for, and enable the analysis of, phenomena as diverse as the formation of Rydberg macrodimers, Penning ionization in dense Rydberg gases, and Rydberg-excitation-blockade effects.
Abstract Coherently manipulating Rydberg atoms in mesoscopic systems has proven challenging due to the unwanted population of nearby Rydberg levels by black-body radiation. Recently, there have been some efforts towards understanding these effects using states with a low principal quantum number that only have resonant dipole–dipole interactions. We perform experiments that exhibit black-body-induced dipole–dipole interactions for a state that also has a significant van der Waals interaction. Using an enhanced rate-equation model that captures some of the long-range properties of the dipolar interaction, we show that the initial degree of Rydberg excitation is dominated by the van der Waals interaction, while the observed linewidth at later times is dominated by the dipole–dipole interaction. We also point out some prospects for quantum simulation.