Entanglement and Interface Conditions on Fields

We consider the vacuum wave function of a free scalar field theory in space partitioned into two regions, with the field obeying Robin conditions (of parameter $\kappa$) on the interface. A direct integration over fields in a subregion is carried out to obtain the reduced density matrix. This leads to a constructive proof of the Reeh-Schlieder theorem. We analyze the entanglement entropy as a function of the Robin parameter $\kappa$. We also consider a specific conditional probability as another measure of entanglement which is more amenable to analysis of the dependence on interface conditions. Finally, we discuss a direct calculation of correlation functions and how it gives an alternate route to the reduced density matrix.