## Abstract

We demonstrate temporal coherent control in two-photon transitions. A pair of broadband ($\mathrm{\Delta}\lambda \sim 20\text{}\mathrm{nm}$), ultrashort ($\mathrm{\Delta}t\sim 100\text{}\mathrm{fs}$), collinear pulses with a variable time delay excites the rubidium atoms into the $5\text{}\mathrm{D}$ state from the $5\text{}\mathrm{S}$ ground state where the two-photon transitions are enhanced by the intermediate level $5\text{}\mathrm{P}$. The excited atoms radiate $5\text{}\mathrm{\mu m}$ ($5\text{}\mathrm{D}\u20136\text{}\mathrm{P}$) and $420\text{}\mathrm{nm}$ ($6\text{}\mathrm{P}\u20135\text{}\mathrm{S}$) light. As a result of tuning the wavelength of the input laser, a superfluorescence at $420\text{}\mathrm{nm}$ exhibits different temporal behaviors. A switching from a beating at the frequency given by the difference between the sequential atomic transitions that involve the $5{\mathrm{P}}_{3/2}$ intermediate level, to a quantum beating due to two different two-photon excitation paths, $5\mathrm{S}\to 5{\mathrm{P}}_{1/2}\to 5\mathrm{D}$ and $5\mathrm{S}\to 5{\mathrm{P}}_{3/2}\to 5\mathrm{D}$ is observed. Based on the simple atom-field interaction theory, an analytic solution, which qualitatively elucidates experimental results, is obtained.

© 2011 Optical Society of America

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