Quantum Separability of the vacuum for Scalar Fields with a Boundary
  Using the Green's function approach we investigate separability of the vacuum state of a massless scalar field with a single Dirichlet boundary. Separability is demonstrated using the positive partial transpose criterion for effective two-mode Gaussian states of collective operators. In contrast to the vacuum energy, entanglement of the vacuum is not modified by the presence of the boundary. 
Jae-Weon Lee, Jaewan Kim, Taeseung Choi