We report a method of two-pathway coherent control using three narrow-band cw laser sources, phase-locked in an optical phase-lock loop, to maintain the high degree of optical coherence required for the coherent control process. In addition, we derive expressions for two-photon transition amplitudes and demonstrate their dependence on the polarization of the field components. This phase-locking technique expands the set of interactions to which coherent control techniques may be applied. It also allows for a constant low-frequency offset between the optical interactions, producing a continuous and constant phase ramp between the interactions, facilitating phase-sensitive detection of the modulating atomic signal. We illustrate this technique with two-photon versus one-photon excitation of a $\mathrm{\Delta }F=1$ component, and alternatively a $\mathrm{\Delta }F=0$ component, of the $6s{}^2{S}_{1/2}\to 7s{}^2{S}_{1/2}$ transition of atomic cesium.

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• Atomic & molecular processes in external fields
• Coherent control