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Physical Review Letters

Long-Range Molecular Resonances in a Cold Rydberg Gas

S. M. Farooqi, D. Tong, S. Krishnan, Jovica Stanojevic, Y. P. Zhang, J. R. Ensher, A. Estrin, Christophe Boisseau, Robin Côté, Edward E. Eyler, P. L. Gould

Published Oct 31, 2003• 118 citations
Publisher

Abstract

We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the 5s ground state to np Rydberg states (n=50-90) reveals resonances at energies corresponding to excited atom pairs (n-1)d+ns. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., approximately 58 000 times the Bohr radius (a(0)) for n=70.

DOI
10.1103/physrevlett.91.183002

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AIP conference proceedings•Jan 1, 2005•1 citation
We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the 5s ground state to np Rydberg states (n=50-90) reveals resonances at energies corresponding to excited atom pairs (n-1)d+ns. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., approximately 58 000 times the Bohr radius (a(0)) for n=70.
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. ^
We have calculated long-range molecular potentials of the 0g+{0}_{g}^{+}0g+​, ${0}_{u}^{\ensuremath{-}}$, and 1u{1}_{u}1u​ symmetries between highly excited rubidium atoms. Strong np+npnp+npnp+np potentials characterized by these symmetries are important in describing interaction-induced phenomena in the excitation spectra of high npnpnp Rydberg states. Long-range molecular resonances are one such phenomenon and they were first reported in S. M. Farooqi et al., Phys. Rev. Lett. 91, 183002 (2003). One class of these resonances occurs at energies corresponding to excited atom pairs $(n\ensuremath{-}1)d+ns$. Such resonances are attributed to l\mathcal{l}l-mixing due to Rydberg-Rydberg interactions so that otherwise forbidden molecular transitions become allowed. We calculate molecular potentials in Hund's case (c), use them to find the resonance line shape, and compare to experimental results.
We report on the observation of ultralong range interactions in a gas of cold rubidium Rydberg atoms. The van der Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii features two important effects: spectral broadening of the resonance lines and suppression of excitation with increasing density. The density dependence of these effects is investigated in detail for the S- and P-Rydberg states with principal quantum numbers n approximately 60 and n approximately 80 excited by narrow-band continuous-wave laser light. The density-dependent suppression of excitation can be interpreted as the onset of an interaction-induced local blockade.
Rydberg molecules, Rydberg-atom or Rydberg-molecule, are an essential ingredient of cold molecular sciences. However, due to the richness of Rydberg-neutral interactions, new kinds of Rydberg molecules and binding mechanisms are still to be discovered. In this work, we predict the existence of ultra-long-range van der Waals trimers in dilute atom-gas mixtures. These are bound states of a Rydberg atom and a diatomic polar molecule mediated by the long-range van der Waals interaction. This new binding mechanism gives rise to trimers with sizes between 5-500~nm and binding energies between 2 MHz and 0.2 kHz depending on the atomic principal quantum number nnn and orbital angular momentum LLL. We show that these molecules can be produced via two-photon photoassociation, with rates on the order of (10−13^{-13}−13 - 10−11^{-11}−11) cm3^{3}3s−1^{-1}−1 for temperatures in the range of (0.5 μKμKμK - 10μKμKμK), and discuss the feasibility of observing trimer resonances.
We review our recent experiments on interaction‐induced effects in a gas of Rydberg atoms excited from a laser‐cooled rubidium cloud. The van‐der‐Waals interaction between a pair of Rydberg atoms separated as far as 100,000 Bohr radii features two important effects: Spectral broadening of the resonance lines and suppression of excitation with increasing Rydberg density. The line broadenings are interpreted in terms of level shifts of Rydberg pairs due to their interaction. The density‐dependent suppression of excitation marks the onset of an interaction‐induced local blockade.