QCMA hardness of ground space connectivity for commuting Hamiltonians

TL;DR

This paper proves that the ground space connectivity problem for commuting local Hamiltonians is QCMA-complete, demonstrating that commuting Hamiltonians can be as computationally complex as general local Hamiltonians.

Contribution

It establishes QCMA-completeness for the commuting local Hamiltonian ground space connectivity problem, a significant step in understanding their computational complexity.

Findings

Ground space connectivity for commuting Hamiltonians is QCMA-complete.

Commuting Hamiltonians can exhibit complexity equivalent to non-commuting cases.

First example showing commuting Hamiltonians have QCMA-hardness similar to general Hamiltonians.

Abstract

In this work we consider the ground space connectivity problem for commuting local Hamiltonians. The ground space connectivity problem asks whether it is possible to go from one (efficiently preparable) state to another by applying a polynomial length sequence of 2-qubit unitaries while remaining at all times in a state with low energy for a given Hamiltonian $H$. It was shown in [Gharibian and Sikora, ICALP'15] that this problem is QCMA-complete for general local Hamiltonians, where QCMA is defined as QMA with a classical witness and BQP verifier. Here we show that the commuting version of the problem is also QCMA-complete. This provides one of the first examples where commuting local Hamiltonians exhibit complexity theoretic hardness equivalent to general local Hamiltonians.