Atomic Clock Using Microwave Pulse-Coherent Techniques

In order to eliminate the light shifts and to increase the stability of gas-cell atomic clocks using optical pumping of the alkali atoms, the detection of a microwave pulse-induced coherent emission is used as the correction signal to lock a crystal oscillator to the atomic transition. Also, the line width of the O-O hyperfine transition is artificially narrowed by subjecting the atoms in the cell to two phase-coherent microwave pulses. In this case the resonance line has a width characteristic of the pulse-repetition frequency which can be made several times smaller than the natural line width in the cell. Proper timing of the sequence of microwave pulses and light pulses for optical pumping is necessary to conserve phase coherence and to obtain a well-defined Ramsey pattern. Results of the tests on a laboratory model using Rubidium-87 atoms show the elimination of the light shifts and an improvement of the short-term stability of the crystal oscillator by one order of magnitude.