Black hole as a multipartite entangler: multi-entropy in AdS3/CFT2

Abstract We study multipartite entanglement in typical pure states holographically dual to pure BTZ black holes, using multi-entropy and its “genuine” version. In the bulk, these quantities are computed by minimal geodesic networks (so-called Steiner trees). We find that at sufficiently high temperature, the genuine tripartite multi-entropy exhibits a volume-law scaling in sharp contrast to vacuum AdS3, where the genuine contribution is universal and size-independent. Moreover, we find another phase: once one subsystem exceeds half of the total system, the leading genuine tripartite entanglement vanishes and reduces to that for global AdS3. This transition is indeed consistent with recent arguments for distillable EPR pairs in tripartite Haar-random states. Motivated by finite-cutoff holography, we further study the radial cutoff dependence of multi-entropy and show that genuine multi-entropy acquires nontrivial size dependence even for the tripartite case in AdS3. As a byproduct, we also observe an intriguing “area-law” contribution to multi-entropy that is relevant to vacuum AdS3.