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Cold atoms in highly excited Rydberg states are promising candidates to implement quantum logic gates of a quantum computer via long-range dipole-dipole interaction [1,2]. Two-qubit gates require a controlled interaction of only two close Rydberg atoms. We report on the first spectroscopic observation of the resonant dipole-dipole interaction at a Stark-tuned Förster resonance between two cold rubidium Rydberg atoms confined in a small laser excitation volume [3].
The experiments were done with cold Rb Rydberg atoms in a magneto-optical trap. An excitation volume of 15-20 microns in size was formed by two tightly focused laser beams in crossed-beam geometry. Single Rydberg atoms were detected with 65% efficiency using selective field ionization and channel electron multiplier that provided the atom-number resolution in combination with the post-selection technique. High-resolution spectra of the Förster resonance Rb(37P,J=3/2)+Rb(37P,J=3/2)-->Rb(37S)+Rb(38S) have been obtained for N=1-5 of the detected Rydberg atoms. The observed resonance amplitude and width grow as N increases. The resonance in spectrum S1 (N=1) must be absent for a perfect detector, but appears due to finite detection efficiency. For precise comparison with theory, we have done the numerical Monte-Carlo simulations for 2-5 interacting Rydberg atoms, initially excited to the 37P(J=3/2,|M|=1/2) state and randomly positioned in a cubic volume of 5-50 microns in size. Dependences of the Förster resonance spectrum on the number of atoms, excitation volume, and interaction time were investigated. A good fit of the experimental data has been obtained at the 0.515 microsecond interaction time and 18x18x18 micron-cubed laser excitation volume. This effective interaction time is less than that we set up in the experiment (3 microseconds), due to hyperfine structure and parasitic electromagnetic fields, which can be taken into account by reducing the interaction time. Good agreement with theory allowed us to identify the numbers of actually interacting Rydberg atoms corresponding to different N. It turned out that S1 and S2 spectra almost completely originate from the interaction of two Rydberg atoms. To the best of our knowledge, this is the first observation of the Stark-tuned Förster resonance for two Rydberg atoms. The method used thus paves the way to a detailed investigation of two-atom interactions, even when the detection efficiency is far below 1. The peculiarities of the obtained results will be discussed in this report. This work was supported by RFBR (Grants No. 09-02-90427 and No. 09-02-92428), by Consortium EINSTEIN, by the Russian Academy of Sciences, and by Dynasty Foundation. [1] D.Jaksh et al., Phys. Rev. Lett., 2000, v.85, p.2208. [2] M.D.Lukin et al., Phys. Rev. Lett., 2001, v.87, p.037901. [3] I.I.Ryabtsev et al., Phys. Rev. Lett., 2010, v.104, p.073003. |
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