Entanglement entropy of $\alpha$-vacua in de Sitter space
  We consider the entanglement entropy of a free massive scalar field in the one parameter family of $\alpha$-vacua in de Sitter space by using a method developed by Maldacena and Pimentel. An $\alpha$-vacuum can be thought of as a state filled with particles from the point of view of the Bunch-Davies vacuum. Of all the $\alpha$-vacua we find that the entanglement entropy takes the minimal value in the Bunch-Davies solution. We also calculate the asymptotic value of the R\'enyi entropy and find that it increases as $\alpha$ increases. We argue these feature stem from pair condensation within the non-trivial $\alpha$-vacua where the pairs have an intrinsic quantum correlation. 
Sugumi Kanno, Jeff Murugan, Jonathan P. Shock, Jiro Soda
hep-th gr-qc