Metastability transfer spectroscopy with two like ions in the same trap

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RESUMO

High-resolution optical spectroscopy of an individual trapped ion is hampered by lack of sharp lasers. This suggests the use of a second metastable excited ion as an ultrasharp light source. To this end, laser-cool two barium ions to an equilibrium distance of ≈8 μm on the z (symmetry) axis of the trap and, in this (earth)(Ba+)2-molecule, visually or photoelectrically identify them as A and B by their location. Briefly turn on a 455-nm spectral lamp until one of the ions, say the A ion, is pumped into the metastable D5/2 level and turns invisible. Focus on the visible, spatially well-resolved B ion and turn off the blue and red illumination lasers for ≈15 s. Then turn them back on again and check on whether the excitation by chance has been transferred to the B ion and is now in the D5/2 level and dark while the A ion is bright. The cross section for absorption of the λ(D5/2 → S1/2) ≡ λ0 = 1.76 μm radiation by a stationary ion can be >λ02/2π. Thus, by pushing the two ions together to ≈λ0/4 by turning on a much stronger trapping field during the excitation exchange period, one might be able to detect excitation transfer in >10% of the attempts. The ions are tuned relative to each other by a 0- to 10-mV/cm variable dc field in the z direction, which displaces them axially and causes them to see different rf fields, which Stark-shifts their frequencies. In this way, a resonant transfer response as sharp as twice the natural width of the D5/2 level, 11 mHz or a Q ≈ 0.4 × 1017, might be demonstrated.

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