  A recent experiment [Lanting et al., PRX, (2014)] claimed to provide evidence
of up to $8$-qubit entanglement in a D-Wave quantum annealing device. However,
entanglement was measured using qubit tunneling spectroscopy, a technique that
provides indirect access to the state of the system at intermediate times
during the anneal by performing measurements at the end of the anneal with a
probe qubit. In addition, an underlying assumption was that the quantum
transverse-field Ising Hamiltonian, whose ground states are already highly
entangled, is an appropriate model of the device, and not some other (possibly
classical) model. This begs the question of whether alternative, classical or
semiclassical models would be equally effective at predicting the observed
spectrum and thermal state populations. To check this, we consider a recently
proposed classical rotor model with classical Monte Carlo updates, which has
been successfully employed in describing features of earlier experiments
involving the device. We also consider simulated quantum annealing with quantum
Monte Carlo updates, an algorithm that samples from the instantaneous Gibbs
state of the device Hamiltonian. Finally, we use the quantum adiabatic master
equation, which cannot be efficiently simulated classically, and which has
previously been used to successfully capture the open system quantum dynamics
of the device. We find that only the master equation is able to reproduce the
features of the tunneling spectroscopy experiment, while both the classical
rotor model and simulated quantum annealing fail to reproduce the experimental
results. We argue that this bolsters the evidence for the reported
entanglement.
