High Contrast Ramsey Fringes with Coherent-Population-Trapping Pulses in a Double Lambda Atomic System

Abstract

We report the observation of Raman-Ramsey fringes using a double lambda scheme creating coherent population trapping in an atomic ensemble combined with pulsed optical radiations. The observation was made in a Cs vapor mixed with ${\mathrm{N}}_{2}$ buffer gas in a closed cell. The double lambda scheme is created with $\mathrm{lin}\ensuremath{\perp}\mathrm{lin}$ polarized laser beams leading to higher contrast than the usual simple lambda scheme. The pulsed trapping technique leads to narrow fringe widths scaling as $1/(2T)$ with high contrasts which are no longer limited by the saturation effect. This technique operates in a different way from the classical Ramsey sequence: the signal is done by applying a long trapping pulse to prepare the atomic state superposition, and fringe detection is accomplished by optical transmission during a short second trapping pulse without any perturbation of the dark state.