In this work, we study the phenomena of quantum entanglement by computing de Sitter entanglement entropy from von Neumann measure. For this purpose we consider a bipartite quantum field theoretic setup in presence of axion originating from ${bf Type~ II~B}$ string theory. We consider the initial vacuum to be CPT invariant non-adiabatic $alpha$ vacua state under ${bf SO(1,4)}$ ismometry, which is characterized by a real one-parameter family. To implement this technique we use a ${bf S^2}$ which divide the de Sitter into two exterior and interior sub-regions. First, we derive the wave function of axion in an open chart for $alpha$ vacua by applying Bogoliubov transformation on the solution for Bunch-Davies vacuum state. Further, we quantify the density matrix by tracing over the contribution from the exterior region. Using this result we derive entanglement entropy, R$acute{e}$nyi entropy and explain the long-range quantum effects in primordial cosmological correlations. We also provide a comparison between the results obtained from Bunch-Davies vacuum and the generalized $alpha$ vacua, which implies that the amount of quantum entanglement and the long-range effects are larger for non zero value of the parameter $alpha$. Most significantly, our derived results for $alpha$ vacua provides the necessary condition for generating non zero entanglement entropy in primordial cosmology.
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