Quantum many-body scars from virtual entangled pairs*

We study weak ergodicity breaking in a one-dimensional, non-integrable spin-1 XY model. We construct for it an exact highly excited eigenstate, which despite having a non-zero energy density, can be represented analytically by a finite bond-dimension matrix product state (MPS) with area-law entanglement. Upon a quench to a finite Zeeman field, the state undergoes periodic dynamics with perfect many-body revivals, in stark contrast to other generic initial states which instead rapidly thermalize. Remarkably, we find that the dynamics can be completely understood in terms of the evolution of entangled virtual spin-1/2 degrees of freedom, which in turn underpin the presence of an O(L) tower of strong-eigenstate thermalization hypothesis (ETH)-violating many-body eigenstates: quantum many-body scars. The scars we find are therefore of novel origin, and we provide insight into their nature and entanglement structure.