where a logarithmic behavior has been found for one interval. In particular $S_A =(c/3)\log(\ell/a)$ where $\ell$ is the length of the interval and $c$ is the central charge of the theory. The method employed to get the analytic result for $S_A$ is the replica trick, which means first to compute $\textrm{Tr} \rho_A^n$ for integer $n$ and then to perform an analytic continuation to real values of $n$ in order to take $S_A= -\,\partial_n \textrm{Tr} \rho_A^n \big|_{n=1}$ [@Callan:1994py; @Holzhey:1994we; @Calabrese:2004eu] (see [@Calabrese:2009qy] for a recent review).\
For quantum field theories with a holographic dual, the problem of computing the entanglement entropy through a bulk description has been addressed in [@Ryu:2006bv; @Ryu:2006ef]. The holographic prescription to obtain $S_A$ associated to a region $A$ in the $d+1$ dimensional boundary theory is the following. On a fixed time slice (see [@Hubeny:2007xt] for a generalization to time dependent backgrounds), among all the $d$ dimensional surfaces extended in the bulk whose boundary coincides with the boundary of $A$, we have to consider the one having minimal area. Denoting this minimal surface by $\gamma_A$, the holographic entanglement entropy is given by $S_A=\textrm{Area}(\gamma_A)/(4 G_N^{(d+2)})$, where $G_N^{(d+2)}$ is the Newton constant of the $d+2$ dimensional theory in the bulk. 