  Motivated by recent experiments in which specific thermal properties of
complex many-body systems were successfully reproduced on a commercially
available quantum annealer, we examine the extent to which quantum annealing
hardware can reliably sample from the thermal state in a specific basis
associated with a target quantum Hamiltonian. We address this question by
studying the diagonal thermal properties of the canonical one-dimensional
transverse-field Ising model on a D-Wave 2000Q quantum annealing processor. We
find that the quantum processor fails to produce the correct expectation values
predicted by Quantum Monte Carlo. Comparing to master equation simulations, we
find that this discrepancy is best explained by how the measurements at finite
transverse fields are enacted on the device. Specifically, measurements at
finite transverse field require the system to be quenched from the target
Hamiltonian to a Hamiltonian with negligible transverse field, and this quench
is too slow. The limitations imposed by such hardware make it an unlikely
candidate for thermal sampling, and it remains an open question what thermal
expectation values can be robustly estimated in general for arbitrary quantum
many-body systems.
