Central Ramsey fringe identification by means of an auxiliary optical field

We propose and demonstrate a simple technique for identifying the central Ramsey fringe of pulsed coherent population trapping resonance. An auxiliary optical field is applied during the free evolution time. It suppresses the nearby fringes but does not change the amplitude of central fringe practically, which marks it out clearly. The theory based on the density matrix equations for the Λ-system configuration of levels and Ramsey interrogation that takes into account the auxiliary optical field is presented and compared to the experiment with 87 Rb atoms. We also propose a technique for improving the middle- and long-term stability of compact atomic clocks.