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Journal of Physics B Atomic Molecular and Optical Physics

Spectroscopy of an ultracold Rydberg gas and signatures of Rydberg–Rydberg interactions

Kilian Singer, M. Reetz‐Lamour, Thomas Amthor, Simon Fölling, M. Tscherneck, Matthias Weidemüller

Published Jan 6, 2005• 32 citations
Publisher

Abstract

We report on experiments on Rydberg–Rydberg interaction-induced effects in a gas of 87 Rb Rydberg atoms. A compact setup for two-photon continuous-wave excitation of high-lying Rydberg states out of an ultracold atomic gas is presented. The performance of the apparatus is characterized by high-resolution spectroscopy of Rydberg states. Signatures of interaction-induced effects are identified by qualitatively analysing the dependence of Rydberg excitation spectra on the intensity and the duration of the second-step laser excitation.

DOI
10.1088/0953-4075/38/2/023

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We report on experiments on Rydberg–Rydberg interaction-induced effects in a gas of 87 Rb Rydberg atoms. A compact setup for two-photon continuous-wave excitation of high-lying Rydberg states out of an ultracold atomic gas is presented. The performance of the apparatus is characterized by high-resolution spectroscopy of Rydberg states. Signatures of interaction-induced effects are identified by qualitatively analysing the dependence of Rydberg excitation spectra on the intensity and the duration of the second-step laser excitation.
Abstract We present experiments on two‐photon excitation of 87Rb atoms to Rydberg states. For this purpose, two continuous‐wave (cw)‐laser systems for both 780 nm and 480 nm have been set up. These systems are optimized to a small linewidth (well below 1 MHz) to get both an efficient excitation process and good spectroscopic resolution. To test the performance of our laser system, we investigated the Stark splitting of Rydberg states. We were able to see for both of the n = 41D finestructure states the electrical field dependent |mj| splitting. To show the ability of spatially selective excitation to Rydberg states, we excited rubidium atoms in an electrical field gradient and investigated both linewidths and lineshifts. Furthermore we were able to excite the atoms selectively from the two hyperfine ground states to Rydberg states. Finally, we investigated the Autler‐Townes splitting of the 5S1/2(F = 2)→5P3/2(F = 3) transition coupled to the light field via a Rydberg state to determine the Rabi frequency of this excitation step.
We report the measurement of the Rydberg excitation spectrum by two-photon process in ultracold 40K^{40}\mathrm{K}40K Fermi gases. Two different methods are employed to measure the Rydberg excitation spectrum, depending on the power of the probe laser. One scheme is to reduce atomic losses by means of electromagnetically induced transparency. The other is to enhance the atomic losses by spontaneous avalanche ionization due to the strong Rydberg-Rydberg interactions. We verify the consistency of both of the methods. The highest Rydberg states detectable in our experiment are limited to $n\ensuremath{\le}62$ due to the competition between the long Rydberg blockade effective range and the limited atomic cloud size.
We present a study of a cold strontium Rydberg gas. The narrowband laser excitation of Rydberg states in the range n = 20–80 from a 6 mK cloud of strontium atoms is detected using the spontaneous ionization of the Rydberg atoms. Using a high-resolution step-scanning technique, we perform detailed measurements of the Stark maps of selected Rydberg states. We find excellent agreement between the measured Stark maps and a numerical calculation based on an independent electron model. Finally we show that excitation of the second valence electron can be used to probe the dynamics of the Rydberg gas with nanosecond temporal resolution via autoionization.
One of the most striking features of the strong interactions between Rydberg atoms is the Rydberg blockade effect, which allows only a single excitation to the Rydberg state within the volume of the blockade sphere. Here we present a method that spatially visualizes this phenomenon in an inhomogeneous gas of ultracold rubidium atoms. In our experiment we scan the position of one of the excitation lasers across the cold cloud and determine the number of Rydberg excitations detected as a function of position. Comparing this distribution to the one obtained for the number of ions created by a two-photon ionization process via the intermediate 5P5P5P level, we demonstrate that the blockade effect modifies the width of the Rydberg excitation profile. Furthermore, we study the dynamics of the Rydberg excitation and find that the time scale for the excitation depends on the atomic density at the beam position.
New phenomena have been observed by exciting ultracold ground-state atoms to Rydberg states. Starting with a dense sample of Rb atoms in a magneto-optical trap, we excite them to high Rydberg states (n = 30 - 90) with a single UV excitation pulse. Long-range molecular resonances with interatomic distances exceeding 104 a0 have been discovered. We have also observed the Rydberg excitation blockade, crucial for the implementation of quantum phase gates using neutral atoms. The physics behind these observations is attributed to the strong long-range van der Waals interactions among the Rydberg atoms. In addition, we have seen dipole (E1) forbidden but quadrupole (E2) allowed 5s → nd transitions, allowing us to determine the E2 oscillator strengths to these high Rydberg states. Our narrow-band UV pulsed laser system, with ∼100 MHz bandwidth, also allows us to measure the ratios of j-dependent oscillator strengths to the p and d Rydberg states. ^
Interaction between Rydberg atoms can significantly modify Rydberg excitation dynamics. Under a resonant driving field the Rydberg-Rydberg interaction in high-lying states can induce shifts in the atomic resonance such that a secondary Rydberg excitation becomes unlikely leading to the Rydberg blockade effect. In a related effect, off-resonant coupling of light to Rydberg states of atoms contributes to the Rydberg anti-blockade effect where the Rydberg interaction creates a resonant condition that promotes a secondary excitation in a Rydberg atomic gas. Here, we study the light-matter interaction and dynamics of off-resonant two-photon excitations and include two- and three-atom Rydberg interactions and their effect on excited state dynamics in an ensemble of cold atoms. In an experimentally-motivated regime, we find the optimal physical parameters such as Rabi frequencies, two-photon detuning, and pump duration to achieve significant enhancement in the probability of generating doubly-excited collective atomic states. This results in large auto-correlation values due to the Rydberg anti-blockade effect and makes this system a potential candidate for a high-purity two-photon Fock state source.