, where 1/2 of the character of |+⟩ is the photonic character |G, 1⟩, and the decoherence rate due to cavity loss is 1/2 of the photonic population decay rate τ -1 c . Fig. presents the ρ +-(t) in a lossy cavity for fixed g c , same as those in Fig. , except with the inclusion of cavity loss in the HEOM simulation. The extracted coherence lifetimes (using Eq. 20) are: (a) T 2 = 46.6 fs for √ N g c = 100 meV , (b) T 2 = 58.9 fs for √ N g c = 141.4 meV, and c) T 2 = 77.6 fs for √ N g c = 200 meV. One can see that Re[ρ +-(t)] indeed decays faster in a lossy cavity compared to a perfect cavity, but coherence between |+⟩ and |-⟩ still lasts much longer compared to the typical value of electronic decoherence rate. For example, when √ N g c = 200 meV (Fig. ) the decoherence time is 77.6 fs when having a cavity loss rate of τ -1 c = 8.83 meV, which is about 3 times longer than the outside cavity case. Thus, the presence of strong collective light-matter coupling still enhances the quantum coherence of the bright polaritonic states even in the presence of cavity loss. Fig. shows the coherence lifetimes T 2 for a finite light-matter detuning ∆ = ω c -(ω x + λ). We consider the coherences in both lossless and lossy cavities, and for lossy cavities we have quality factors from Q = 167 to Q = 1500 that are representative of experimentally realizable optical cavities. For positive ∆, T 2 increases with increasing ∆ until it reaches a turnover point where T 2 decreases with further increases in ∆. From our FGR analysis, this turnover is caused by the competition between the population transfer from |+⟩ → {|D k ⟩} given by the rate k +→D , and the population transfer from |-⟩ → {|D k ⟩} given by the rate k -→D . We also include the photonic loss to the |G, 1⟩ state from the |±⟩ states. Combining all contributions to the decoherence rate, we have the second key result of this letter