We demonstrate here the direct observation of quantum jumps between the $6^2$${\mathrm{S}}_{1/2}$ state and the $5^2$${\mathrm{D}}_{5/2}$ state of an individual laser-cooled ${\mathrm{Ba}}^{+}$ ions contained in a radio-frequency trap. The state detection and cooling are performed by two lasers which cause $6^2$${\mathrm{S}}_{1/2}$${\mathrm{-}6}^2$${\mathrm{P}}_{1/2}$${\mathrm{-}5}^2$ ${\mathrm{D}}_{3/2}$ transitions. Incoherent excitation to the $5^2$${\mathrm{D}}_{5/2}$ state (via the $6^2$${\mathrm{P}}_{3/2}$ level) causes the fluorescence from the $6^2$${\mathrm{P}}_{1/2}$ 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.

• Received 5 May 1986

DOI:https://doi.org/10.1103/PhysRevLett.56.2797