Observation of Quantum Jumps in a Single Atom

Abstract

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 $^{2}S_{\frac{1}{2}}\ensuremath{\rightarrow}^{2}P_{\frac{1}{2}}$ first resonance line. When the ion "jumps" back from the metastable $D$ state to the ground $S$ state, the $S\ensuremath{\rightarrow}P$ 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.