Noncontextual Pauli Hamiltonians

TL;DR

This paper rigorously analyzes noncontextual Pauli Hamiltonians, showing they can describe more interactions than diagonal ones, have efficiently classically describable eigenspaces, and enable new classes of simulatable quantum states.

Contribution

It establishes fundamental properties of noncontextual Pauli Hamiltonians, including their composition, eigenspace structure, and potential for efficient classical simulation.

Findings

Noncontextual Hamiltonians can include more Pauli operators than diagonal Hamiltonians.

Eigenspaces of these Hamiltonians admit efficient classical descriptions.

Degeneracies in the eigenspectrum can be characterized, enabling new simulatable states.

Abstract

Contextuality is a key feature of quantum mechanics, and identification of noncontextual subtheories of quantum mechanics is of both fundamental and practical importance. Recently, noncontextual Pauli Hamiltonians have been defined in the setting of variational quantum algorithms. In this work we rigorously establish a number of properties of noncontextual Pauli Hamiltonians. We prove that these Hamiltonians can be composed of more Pauli operators than diagonal Hamiltonians. This establishes that noncontextual Hamiltonians are able to describe a greater number of physical interactions. We then show that the eigenspaces admit an efficient classical description. We analyse the eigenspace of these Hamiltonians and prove that for every eigenvalue there exists an associated eigenvector whose stabilizer rank scales linearly with the number of qubits. We prove that further structure in these Hamiltonians allow us to derive where degeneracies in the eigenspectrum can arise. We thus open the field to a new class of efficiently simulatable states.