Entanglement Hamiltonian of the quantum N\'eel state

Abstract

Two-dimensional Projected Entangled Pair States (PEPS) provide a unique framework giving access to detailed entanglement features of correlated (spin or electronic) systems. For a bi-partitioned quantum system, it has been argued that the Entanglement Spectrum (ES) is in a one-to-one correspondence with the physical edge spectrum on the cut and that the structure of the corresponding Entanglement Hamiltonian (EH) reflects closely bulk properties (finite correlation length, criticality, topological order, etc...). However, entanglement properties of systems with spontaneously broken continuous symmetry are still not fully understood. The spin-1/2 square lattice Heisenberg antiferromagnet provides a simple example showing spontaneous breaking of SU(2) symmetry down to U(1). The ground state can be viewed as a "quantum N\'eel state" where the classical (N\'eel) staggered magnetization is reduced by quantum fluctuations. Here I consider the (critical) Resonating Valence Bond state doped with spinons to describe such a state, that enables to use the associated PEPS representation (with virtual bond dimension $D=3$) to compute the EH and the ES for a partition of an (infinite) cylinder. In particular, I find that the EH is (almost exactly) a chain of a dilute mixture of heavy ($\downarrow$ spins) and light ($\uparrow$ spins) hardcore bosons, where light particles are subject to long-range hoppings. The corresponding ES shows drastic differences with the typical ES obtained previously for ground states with restored SU(2)-symmetry (on finite systems).