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Journal of Physics Conference Series

Exploiting Rydberg blockade to probe strongly-coupled Rydberg atom pairs

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

Published Sep 7, 2015• 0 citations
Publisher

Abstract

Rydberg blockade at very-high- n , n ∼ 300, is examined using strontium n 1 F 3 Rydberg atoms excited in a small volume defined by two crossed tightly-focused laser beams. Strong but not complete blockade is observed and discussed with the aid of quantum calculations using a two-active-electron model. Nonetheless, the probability for creating one, and only one, Rydberg atom is large, >0.6, sufficient to enable study of strongly- coupled Rydberg atom pairs.

DOI
10.1088/1742-6596/635/9/092022

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Rydberg blockade at very-high- n , n ∼ 300, is examined using strontium n 1 F 3 Rydberg atoms excited in a small volume defined by two crossed tightly-focused laser beams. Strong but not complete blockade is observed and discussed with the aid of quantum calculations using a two-active-electron model. Nonetheless, the probability for creating one, and only one, Rydberg atom is large, >0.6, sufficient to enable study of strongly- coupled Rydberg atom pairs.
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 production of very-high-nnn ($n\ensuremath{\sim}300$--500) strontium Rydberg atoms is explored using a crossed-laser-atom-beam geometry. nnn1S0{}^{1}{S}_{0}1S0​ and nnn1D2{}^{1}{D}_{2}1D2​ states are created by two-photon excitation via the 5s5p5s5p5s5p 1P1{}^{1}{P}_{1}1P1​ intermediate state using radiation with wavelengths of $\ensuremath{\sim}$461 and $\ensuremath{\sim}$413 nm. Rydberg atom densities as high as $\ensuremath{\sim}3\ifmmode\times\else\texttimes\fi{}{10}^{5}$ cm${}^{\ensuremath{-}3}$ have been achieved, sufficient that Rydberg-Rydberg interactions can become important. The isotope shifts in the Rydberg series limits are determined by tuning the 461-nm light to preferentially excite the different strontium isotopes. Photoexcitation in the presence of an applied electric field is examined. The initially quadratic Stark shift of the nnn1P1{}^{1}{P}_{1}1P1​ and nnn1D2{}^{1}{D}_{2}1D2​ states becomes near-linear at higher fields and the possible use of n1D2n{}^{1}{D}_{2}n1D2​ states to create strongly polarized, quasi-one-dimensional electronic states in strontium is discussed. The data are analyzed with the aid of a two-active-electron (TAE) approximation. The two-electron Hamiltonian, within which the Sr2+{}^{2+}2+ core is represented by a semi-empirical potential, is numerically diagonalized allowing the calculation of the energies of high-nnn Rydberg states and their photoexcitation probabilities.
The dipole blockade of very-high-$n, n\ensuremath{\sim}300$, 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.
Dipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.