Entanglement entropy and D1-D5 geometries

In conformal field theories (CFTs) with a gravitational anti-de Sitter (AdS) dual it is possible to calculate the entanglement entropy of a region A holographically by using the Ryu-Takayanagi formula. In this work we consider systems that are in a pure state that is not the vacuum. We study in particular the two-dimensional conformal field theory dual to type IIB string theory on AdS(3) x S-3 x T-4 and focus on the 1/4-BPS states described holographically by the two-charge microstate geometries. We discuss a general prescription for the calculation of the entanglement entropy in these geometries that are asymptotically AdS(3) x S-3. In particular we study analytically the perturbative expansion for a single, short interval: we show that the first nontrivial terms in this expansion are consistent with the expected CFT structure and with previous results on the vacuum expectation values of chiral primary operators for the 1/4-BPS configurations.