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We demonstrate here the direct observation of quantum jumps between the 6^2S₁/₂ state and the 5^2D₅/₂ state of an individual laser-cooled Ba^+ ions contained in a radio-frequency trap. The state detection and cooling are performed by two lasers which cause 6^2S₁/₂-6^2P₁/₂-5^2 D₃/₂ transitions. Incoherent excitation to the 5^2D₅/₂ state (via the 6^2P₃/₂ level) causes the fluorescence from the 6^2P₁/₂ state to be suppressed for >30-sec lifetime of that state, after which the fluorescence reappears. The resulting ``telegraph signal'' provides a direct monitor of the atomic state.
Nagourney et al. (Mon,) studied this question.