  We present \emph{multi-qubit correction} (MQC) as a novel postprocessing
method for quantum annealers that views the evolution in an open-system as a
Gibbs sampler and reduces a set of excited states to a new synthetic state with
lower energy value. After sampling from the ground state of a given (Ising)
Hamiltonian, MQC compares pairs of excited states to recognize virtual
tunnels--i.e., a group of qubits that changing their states simultaneously can
result in a new state with lower energy value--and successively converges to
the ground state. Experimental results using D-Wave 2000Q quantum annealers
demonstrate that MQC finds samples with notably lower energy values and
improves the reproducibility of results when compared to recent
hardware/software advances in the realm of quantum annealing, such as
spin-reversal transforms, classical postprocessing techniques, and increased
inter-sample delay between successive measurements.
