Complete Characterization of the Ground Space Structure of Two-Body Frustration-Free Hamiltonians for Qubits

TL;DR

This paper provides a complete description of the ground space structure of two-body frustration-free qubit Hamiltonians, revealing it as a span of tree tensor network states, and relates the degeneracy problem to classical complexity.

Contribution

It offers a full characterization of the ground space as a span of tree tensor network states, advancing understanding of their structure and complexity.

Findings

Ground space is a span of tree tensor network states with the same tree structure.

Determining ground state degeneracy has the same complexity as its classical analog.

The ground space structure can be fully characterized for any two-body frustration-free Hamiltonian.

Abstract

The problem of finding the ground state of a frustration-free Hamiltonian carrying only two-body interactions between qubits is known to be solvable in polynomial time. It is also shown recently that, for any such Hamiltonian, there is always a ground state that is a product of single- or two-qubit states. However, it remains unclear whether the whole ground space is of any succinct structure. Here, we give a complete characterization of the ground space of any two-body frustration-free Hamiltonian of qubits. Namely, it is a span of tree tensor network states of the same tree structure. This characterization allows us to show that the problem of determining the ground state degeneracy is as hard as, but no harder than, its classical analog.