Lack of symmetry restoration after a quantum quench: An entanglement asymmetry study

We consider the quantum quench in the XX spin chain starting from a tilted Neel state which explicitly breaks the U(1) symmetry of the post-quench Hamiltonian. Very surprisingly, the U(1) symmetry is not restored at large time because of the activation of a nonAbelian set of charges which all break it. The breaking of the symmetry can be effectively and quantitatively characterised by the recently introduced entanglement asymmetry. By a combination of exact calculations and quasi-particle picture arguments, we are able to exactly describe the behaviour of the asymmetry at any time after the quench. Furthermore we show that the stationary behaviour is completely captured by a non-Abelian generalised Gibbs ensemble. While our computations have been performed for a noninteracting spin chain, we expect similar results to hold for the integrable interacting case as well because of the presence of non-Abelian charges also in that case.