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=10\gamma_ab_1, 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.