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Physical Review A

Rydberg blockade effects at n∼300 in strontium

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

Published Nov 23, 2015• 12 citations
Publisher

Abstract

Rydberg blockade at $n\ensuremath{\sim}300$, is examined using strontium $n\phantom{\rule{0.16em}{0ex}}^{1}F_{3}$ 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 $F$ 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-$n$ Rydberg systems under carefully controlled conditions.

DOI
10.1103/physreva.92.051402

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Rydberg blockade at n\ensuremath∼300n\ensuremath{\sim}300n\ensuremath∼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∼0.1\ensuremath{\sim}0.1\ensuremath∼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.
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.
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.
Rydberg atoms are increasingly being used as a means of introducing long-range, tunable interactions into atomic systems through their strong dipole-dipole interactions. When creating such interactions, however, there are a variety of potential effects that can occur, such as state (n and/or L) changing or Penning ionization, which can complicate the dynamics of the system. In the following work, such interactions are investigated for Rydberg-Rydberg collisions using very high-n, n ~300, strontium Rydberg atoms in an atomic beam configuration. By exploiting Rydberg blockade and the inherent distribution of atomic velocities present in an atomic beam, it is possible to create a 1D array of moving Rydberg atoms which will naturally overtake each other over time. Measurements of both Penning ionization and state changing for such collisions are made and compared with classical trajectory Monte-Carlo (CTMC) simulations, with both indicating large collisional cross sections on the order of 1 x 10-6 cm2 for Penning ionization and nearly 8 x 10-5 cm2 for state changing interactions. In a second series of experiments, previous work involving doubly-excited Rydberg states is expanded upon by investigating autoionization with high-n, L > 3, Rydberg states. Past research involving autoionization has shown that the effect is strongly L-dependent, with high-L states having significantly smaller autoionizing rates. To create well-defined, L > 3 states, the use of microwave fields to drive various high-n, n ~ 150, n1F3 → (n+1)1G4, (n+2)1H5 transitions is investigated, with spectroscopic measurements of these transitions providing quantum defect values for n1G4 and n1H5 states. These measurements provide a basis for future studies into the L-dependence of high-n, n ~ 300, autoionizing Rydberg atoms, with the eventual goal of creating a long-lived, doubly-excited “planetary” Rydberg state.