Observation of Quantum Jumps in a Single Atom

We detect the radiatively driven electric quadrupole transition to the metastable ${}_{}{}^2{}_{}{}^{}D_{\frac{5}{2}}^{}{}_{}{}^{}$ state in a single, laser-cooled Hg II ion by monitoring the abrupt cessation of the fluorescence signal from the laser-excited ${}_{}{}^2{}_{}{}^{}S_{\frac{1}{2}}^{}{}_{}{}^{}\to {}_{}{}^2{}_{}{}^{}P_{\frac{1}{2}}^{}{}_{}{}^{}$ first resonance line. When the ion "jumps" back from the metastable $D$ state to the ground $S$ state, the $S\to P$ resonance fluorescence signal immediately returns. The statistical properties of the quantum jumps are investigated; for example, photon antibunching in the emission from the $D$ state is observed with 100% efficiency.