Precise measurement of hyperfine intervals using avoided crossing of   dressed states
  We demonstrate a technique for precisely measuring hyperfine intervals in alkali atoms. The atoms form a three-level $\Lambda$ system in the presence of a strong control laser and a weak probe laser. The dressed states created by the control laser show significant linewidth reduction. We have developed a technique for Doppler-free spectroscopy that enables the separation between the dressed states to be measured with high accuracy even in room-temperature atoms. The states go through an avoided crossing as the detuning of the control laser is changed from positive to negative. By studying the separation as a function of detuning, the center of the level-crossing diagram is determined with high precision, which yields the hyperfine interval. Using room-temperature Rb vapor, we obtain a precision of 44 kHz. This is a significant improvement over the current precision of ~ 1 MHz. 
Umakant D. Rapol and Vasant Natarajan