Observation of subnatural-linewidth spectra in cold Li 6 atoms

We report an observation of the subnatural-linewidth spectroscopy in cold Li6 atoms by combining the interference contrast method and the time-delayed coincidence technique. The measured linewidth of D1 lines is as narrow as 3.6 MHz, which is well below its natural linewidth of 5.8 MHz. The hyperfine splitting of the 2P1/2 state is measured as 26.112(20) MHz with this subnatural-linewidth spectroscopy. The transitions of the D2 lines are partially resolved by using the developed technique. The time-delayed coincidence technique based on the interference contrast method in cold atoms provides a route for robust fluorescence measurements and determination of unresolved transitions.