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Physical Review A

Rydberg blockade in a hot atomic beam

S. Yoshida, Joachim Burgdörfer, X. Zhang, F. B. Dunning

Published Apr 11, 2017• 6 citations
Publisher

Abstract

The dipole blockade of very-high-$n, n\ensuremath{\sim}300$, strontium $5snf{\phantom{\rule{0.16em}{0ex}}}^{1}{F}_{3}$ Rydberg atoms in a hot atomic beam is studied. For such high $n$, the blockade radius can exceed the linear dimensions of the excitation volume. Rydberg atoms formed inside the excitation volume can, upon leaving the region, continue to suppress excitation until they have moved farther away than the blockade radius. Moreover, the high density of states originating from the many magnetic sublevels associated with the $F$ states results in a small but finite probability of excitation of $L=3\phantom{\rule{0.28em}{0ex}}n{\phantom{\rule{0.16em}{0ex}}}^{1}{F}_{3}$ atom pairs at small internuclear separations below the blockade radius. We demonstrate that these effects can be distinguished from one another by the distinct features they imprint on the Mandel $Q$ parameter as a function of the duration of the exciting laser.

DOI
10.1103/physreva.95.042705

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The dipole blockade of very-high-n,n\ensuremath∼300n, n\ensuremath{\sim}300n,n\ensuremath∼300, strontium 5snf1F35snf{\phantom{\rule{0.16em}{0ex}}}^{1}{F}_{3}5snf1F3​ Rydberg atoms in a hot atomic beam is studied. For such high nnn, the blockade radius can exceed the linear dimensions of the excitation volume. Rydberg atoms formed inside the excitation volume can, upon leaving the region, continue to suppress excitation until they have moved farther away than the blockade radius. Moreover, the high density of states originating from the many magnetic sublevels associated with the FFF states results in a small but finite probability of excitation of L=3n1F3L=3\phantom{\rule{0.28em}{0ex}}n{\phantom{\rule{0.16em}{0ex}}}^{1}{F}_{3}L=3n1F3​ atom pairs at small internuclear separations below the blockade radius. We demonstrate that these effects can be distinguished from one another by the distinct features they imprint on the Mandel QQQ parameter as a function of the duration of the exciting laser.
Rydberg blockade at $n\ensuremath{\sim}300$, is examined using strontium n1F3n\phantom{\rule{0.16em}{0ex}}^{1}F_{3}n1F3​ Rydberg atoms excited in an atomic beam in a small volume defined by two tightly focused crossed laser beams. The observation of blockade for such states is challenging due to their extreme sensitivity to stray fields and the many magnetic sublevels associated with FFF states which results in a high local density of states. Nonetheless, with a careful choice of laser polarization to selectively excite only a limited number of these sublevels, sizable blockade effects are observed on an $\ensuremath{\sim}0.1$ mm length scale extending blockade measurements into the near-macroscopic regime and enabling study of the dynamics of strongly coupled many-body high-nnn Rydberg systems under carefully controlled conditions.
Highly excited Rydberg states, neutral atomic states with one or more electrons promoted to states of large principal quantum number n are now commonly used for producing quantum entanglement. This is facilitated through the "Rydberg Blockade" effect where the presence of a single Rydberg atom blocks excitation of further atoms in a surrounding mesoscopic volume due to strong atom-atom interactions induced by the large dipole moments associated with Rydberg atoms. In this work, we report the first observation of this phenomenon in a hot atomic beam using very high n, n ~300 - 500, strontium nF Rydberg states. Inside the blockaded volume (size around 0.1mm), the Rydberg number distribution is sub-Poissonian. However, as predicted by theoretical calculations, due to the anisotropic nature of the atom-atom interactions, the blockade is incomplete. However, the blockade effect is sufficiently strong to enable detailed study of few-body atom-atom interactions and of entanglement. Doubly-excited Rydberg states having a pair of electrons excited to high lying energy levels are typically difficult to create and are short lived because of "autoionization" resulting from electron-electron scattering. Strontium Rydberg atoms are used to study this scattering and we demonstrate an efficient mechanism to eliminate "autoionization" pointing to the production of long-lived strongly correlated two-electron-excited atomic states.
The destruction of high-$n, n\ensuremath{\sim}300$, strontium atoms contained in a hot atomic beam through Rydberg-Rydberg collisions is examined. The Rydberg atoms are initially created, under blockade conditions, in a localized volume and their subsequent motions lead to creation of a string of Rydberg atoms. The Rydberg atoms, however, are formed with a thermal distribution of velocities resulting in Rydberg-Rydberg collisions which lead to their destruction. The experimental data are interpreted using classical trajectory Monte Carlo techniques that simulate the excitation of the Rydberg atoms together with their subsequent motions. Using calculated collisional ionization cross sections and blockade radii, the model yields results in good agreement with experiment. The results highlight the important role collisional destruction can play when attempting to study (long-range) Rydberg-Rydberg interactions in a hot atomic beam.
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.
We investigate the excitation of a collection of cold atoms to Rydberg states. By a direct numerical solution of Schrödinger's equation, we are able to compute various interesting properties of the many-body wavefunction. The high polarizability of Rydberg atoms allows them to support large dipole moments which in turn can interact with each other over long ranges. If the interaction energy between excited atoms is large enough, the resultant energy shift will move the two excitation states out of resonance, thus effectively blocking a two excitation state from occurring. One particular topic investigated is the quantum phase gate, where both groups of atoms are within a blockade radius R b and subjected to a π − 2π − π sequence of pulses. We examine the regime where the groups are neither totally within nor totally outside the blockade radius. Our results explore the tolerance in variation of intergroup lattice distances for a series of quantum gates.