iPEPS failure to represent corner-dependent entanglement in conformal systems

Abstract: Tensor networks, particularly the infinite projected entangled pair state (iPEPS) ansatz, have been shown to represent a vast range of quantum states. While entanglement constraints limit the representation of critical states in one-dimensional iPEPS (commonly referred to as infinite matrix product states (iMPS)), it was previously unclear whether similar limitations exist for two-dimensional iPEPS-represented states. In this work, we investigate a subleading entanglement contribution stemming from a sharp corner in the bipartition and analyze its manifestation in iPEPS. For gapped systems, the corner-dependent term is predicted by the continuum theory to have a particular opening-angle dependence and not to scale with the system size. This is shown to emerge from the iPEPS structure upon averaging over the corner's orientation with respect to the iPEPS lattice orientation. However, for conformal (critical) systems, the predicted corner term scales logarithmically with the system size. This behavior is shown to be impossible in iPEPS, again by arguments relying on the iPEPS structure. We also verify numerically their validity for a particular critical system, the Rokhsar-Kivelson state. We discuss two possible interpretations: First, iPEPS may inherently fail to capture this subleading entanglement feature, potentially indicating additional limitations in their ability to represent other properties of critical systems. Alternatively, the corner-dependent term might vanish for lattice systems, arising only in continuous conformal systems. We discuss the possible implications of this result.