  Quantum annealers require accurate control and optimized operation schemes to
reduce noise levels, in order to eventually demonstrate a computational
advantage over classical algorithms. We study a high coherence four-junction
capacitively shunted flux qubit (CSFQ), using dispersive measurements to
extract system parameters and model the device. Josephson junction asymmetry
inherent to the device causes a deleterious nonlinear cross-talk when annealing
the qubit. We implement a nonlinear annealing path to correct the asymmetry
in-situ, resulting in a substantial increase in the probability of the qubit
being in the correct state given an applied flux bias. We also confirm the
multi-level structure of our CSFQ circuit model by annealing it through small
spectral gaps and observing quantum signatures of energy level crossings. Our
results demonstrate an anneal-path correction scheme designed and implemented
to improve control accuracy for high-coherence and high-control quantum
annealers, which leads to an enhancement of success probability in annealing
protocols.

Quantum Physics