Fig. presents Re[ρ +-(t)], the real part of the coherence between the |+⟩ and |-⟩ states, in a lossless cavity (no photon decay). Here, we fix the number of molecules N = 10, and the collective coupling strength √ N g c varies from 100 meV to 200 meV by changing g c . The black solid line corresponds to the coherence under the limit of g c = 0 + (outside the cavity), where ρ +-(t) decays with a Gaussian profile which is consistent with the established result of Gaussian coherence decay. One can see that an increase in √ N g c can significantly prolong the coherence time. An interesting feature we observed is that ρ +-(t) switches from a Gaussian decay to an exponential decay (Markovian limit). To extract the coherence lifetimes τ , we fit Re[ρ +-(t)] to the product of a cosine function and a single exponential decay function