Pivot Hamiltonians as generators of symmetry and entanglement

Abstract

It is well-known that symmetry-protected topological (SPT) phases can be obtained from the trivial phase by an entangler, a finite-depth unitary operator UU. Here, we consider obtaining the entangler from a local ‘pivot’ Hamiltonian H_\text{pivot}Hpivot such that U = e^{i\pi H_\text{pivot}}U=eiπHpivot. This perspective of Hamiltonians pivoting between the trivial and SPT phase opens up two new directions: (i) Since SPT Hamiltonians and entanglers are now on the same footing, can we iterate this process to create other interesting states? (ii) Since entanglers are known to arise as discrete symmetries at SPT transitions, under what conditions can this be enhanced to U(1)U(1) pivot symmetry generated by H_\text{pivot}Hpivot? In this work we explore both of these questions. With regard to the first, we give examples of a rich web of dualities obtained by iteratively using an SPT model as a pivot to generate the next one. For the second question, we derive a simple criterion for when the direct interpolation between the trivial and SPT Hamiltonian has a U(1)U(1) pivot symmetry. We illustrate this in a variety of examples, assuming various forms for H_\text{pivot}Hpivot, including the Ising chain, and the toric code Hamiltonian. A remarkable property of such a U(1)U(1) pivot symmetry is that it shares a mutual anomaly with the symmetry protecting the nearby SPT phase. We discuss how such anomalous and non-onsite U(1)U(1) symmetries explain the exotic phase diagrams that can appear, including an SPT multicritical point where the gapless ground state is given by the fixed-point toric code state.