Observation of the Mollow Triplet from an optically confined single atom

Resonance fluorescence spectrum of a two-level system consists of a single peak that evolves into a triplet structure, known as Mollow triplet, when it is driven by a radiation field above its saturation intensity. Particularly, photons originating from different peaks of the triplet show distinct photon correlations, allowing the fluorescence to be engineered as a useful light source for quantum information processing purposes. We experimentally study the fluorescence spectrum of an optically trapped single Rb-87 atom by exciting a closed two-level transition with a near-resonant laser at different powers. The second-order intensity correlation measurement demonstrates the photon anti-bunching characteristic from a single atom emission as well as the Rabi oscillation of the atom. We also measured the cross correlation between photons coming from the two opposite sidebands of the fluorescence spectrum when an off-resonant field is applied to the atom. The asymmetry in the timing correlation indicates that there is a preferred time-ordering for the photon emission process in the two sidebands. The cascaded generation of time-correlated fluorescence photons with a tunable frequency difference will be useful for quantum optics experiments and quantum communication protocols.