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arXiv:1911.06397 · Journal of Physics B Atomic Molecular and Optical Physics

Depletion imaging of Rydberg atoms in cold atomic gases

M Ferreira-Cao, V Gavryusev, T Franz, R Ferracini Alves, Adrien Signoles, G Zürn, Matthias Weidemüller

Published Feb 7, 2020• 10 citations
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Abstract

Abstract We present a depletion imaging technique to map out the spatial and temporal dependency of the density distribution of an ultracold gas of Rydberg atoms. Locally resolved absorption depletion, observed through differential ground state absorption imaging of a 87 Rb cloud in presence and absence of pre-excited Rydberg atoms, reveals their projected two-dimensional distribution. By employing a closed two-level optical transition uncoupled from the Rydberg state, the highly excited atoms are preserved during imaging. We measure the excitation dynamics of the ∣ 48 S 〉 state of 87 Rb, observing a saturation of the two-dimensional Rydberg density. Such outcome can be explained by the Rydberg blockade effect which prevents resonant excitation of close-by Rydberg atoms due to strong dipolar interactions. By combining the superatom description, where atoms within a blockade radius are represented as collective excitations, with a Monte Carlo sampling, we can quantitatively model the observed excitation dynamics and infer the full three-dimensional distribution of Rydberg atoms, that can serve as a starting point for quantum simulation of many-body dynamics involving Rydberg spin systems.

DOI
10.1088/1361-6455/ab7427

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Abstract We present a depletion imaging technique to map out the spatial and temporal dependency of the density distribution of an ultracold gas of Rydberg atoms. Locally resolved absorption depletion, observed through differential ground state absorption imaging of a 87 Rb cloud in presence and absence of pre-excited Rydberg atoms, reveals their projected two-dimensional distribution. By employing a closed two-level optical transition uncoupled from the Rydberg state, the highly excited atoms are preserved during imaging. We measure the excitation dynamics of the ∣ 48 S 〉 state of 87 Rb, observing a saturation of the two-dimensional Rydberg density. Such outcome can be explained by the Rydberg blockade effect which prevents resonant excitation of close-by Rydberg atoms due to strong dipolar interactions. By combining the superatom description, where atoms within a blockade radius are represented as collective excitations, with a Monte Carlo sampling, we can quantitatively model the observed excitation dynamics and infer the full three-dimensional distribution of Rydberg atoms, that can serve as a starting point for quantum simulation of many-body dynamics involving Rydberg spin systems.
This thesis investigates how a single highly excited atom, called Rydberg atom, can be optically imaged. Direct detection methods based on the scattering of light are hardly applicable due to the small scattering rate of the ground to Rydberg state transition. Instead, a cloud of ground state atoms, normally absorptive, is rendered transparent using electromagnetically induced transparency (EIT), involving an auxiliary probe Rydberg state. The cloud acts as a contrast medium, whose optical response is locally perturbed by the strong Rydberg-Rydberg interaction between the probe and the Rydberg impurity which we want to detect. This perturbation restores absorption within a small volume around the impurity, readily detected and \nspatially resolved on a camera. We call this technique Interaction Enhanced Imaging (IEI). To implement IEI we characterize the optical response of the EIT contrast \nmedium in absence of interactions. By combining measurements of the spatially resolved optical spectrum and of the total Rydberg atom number, we can reconstruct \nthe full one-body density matrix of the three-level system. Next, we excite |nS> or |nP> states and, using IEI, we demonstrate spatially resolved imaging, enabling us \nto study dipolar energy transport. To reach single impurity sensitivity we investigate our current detection fidelity and characterize the signal and noise contributions in \nIEI. We model our interacting system, finding good agreement with experimental data. Based on this model, we predict combinations of Rydberg states for which single-shot single impurity sensitivity should be possible in future experiments.
When cold atoms are laser-excited to Rydberg states, the strong interactions between Rydberg excitations can lead to complex many-body entanglement in the system. The excitation process results in spatial correlations between excitations in the system. In most cases, Rydberg excitation positions are anticorrelated such that no two excitations are within a "blockade radius" of each other. Such systems are well-suited for studying basic many-body physics, as well as for future technological applications such as quantum computation. This thesis describes an experimental apparatus we constructed to perform spatially-sensitive detection of Rydberg atoms. We use the apparatus to perform measurements of the Rydberg-Rydberg pair-correlation function. This results in the first direct spatial images of the Rydberg blockade effect. We measure the blockade radius for a variety of S and D Rydberg states in rubidium. We investigate the dependence of the blockade radius on laser detunings and energy level shifts induced by optical potentials due to trapping lasers. Our results have some implications for atom traps used for neutral atom quantum computation. We also present simulations results used to characterize the performance of the imaging system.
We propose a new all-optical method to image individual Rydberg atoms embedded within dense gases of ground state atoms. The scheme exploits interaction-induced shifts on highly polarizable excited states of probe atoms, which can be spatially resolved via an electromagnetically induced transparency resonance. Using a realistic model, we show that it is possible to image individual Rydberg atoms with enhanced sensitivity and high resolution despite photon-shot noise and atomic density fluctuations. This new imaging scheme could be extended to other impurities such as ions, and is ideally suited to equilibrium and dynamical studies of complex many-body phenomena involving strongly interacting particles. As an example we study blockade effects and correlations in the distribution of Rydberg atoms optically excited from a dense gas.
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.
Cold atomic gases resonantly excited to Rydberg states can exhibit strong optical nonlinearity at the single-photon level. We observe that in such samples radiation trapping leads to an additional mechanism for Rydberg excitation. Conversely we demonstrate that Rydberg excitation provides an in situ probe of the spectral, statistical, temporal, and spatial properties of the trapped rescattered light. We also show that absorption can lead to an excitation saturation that mimics the Rydberg blockade effect. Collective effects due to multiple scattering may coexist with cooperative effects due to long-range interactions between the Rydberg atoms, adding a new dimension to quantum optics experiments with cold Rydberg gases.
Ultracold atomic gases excited to strongly interacting Rydberg states are a promising system for quantum simulations of many-body systems. The dipole blockade of Rydberg excitations is a hallmark of the strong interactions between atoms in these high-lying quantum states. We have measured the Rydberg excitation for rubidium ultracold atoms in magneto-optical traps and for Bose-Einstein condensates loaded into quasi one-dimensional traps. One of the consequences of the dipole blockade is the suppression of fluctuations in the counting statistics of Rydberg excitations. We have obtained experimental results on the dynamics and the counting statistics of Rydberg excitations of ultra-cold Rubidium atoms both on and off resonance, which exhibit sub- and super-Poissonian counting statistics, respectively. We have found strongly bimodal counting distributions in the offresonant regime.