Sub-natural linewidth of the probe absorption spectrum in a cold Cs atom molecule system

We theoretically analysed the linewidth of the probe absorption spectrum in a cold Cs atom--molecule system. The tunnelling coupling between the two excited molecular states, especially for the Cs atom--molecule system, plays an important role in obtaining the sub-natural linewidth of the probe absorption spectrum. For example, when the tunnelling couple strength fulfils \sigma _12=10γ _ab₁, the linewidth is only about 0.66~MHz. Moreover, since the linewidth of interest is dominated by the tunnelling coupling, the absorption peak becomes very narrow even in the case of large pump laser intensities.