Abstract
Many interesting but practically intractable problems can be reduced to
that of finding the ground state of a system of interacting spins;
however, finding such a ground state remains computationally difficult.
It is believed that the ground state of some naturally occurring spin
systems can be effectively attained through a process called quantum
annealing. If it could be harnessed, quantum annealing might improve on
known methods for solving certain types of problem. However, physical
investigation of quantum annealing has been largely confined to
microscopic spins in condensed-matter systems. Here we use quantum
annealing to find the ground state of an artificial Ising spin system
comprising an array of eight superconducting flux quantum bits with
programmable spin-spin couplings. We observe a clear signature of
quantum annealing, distinguishable from classical thermal annealing
through the temperature dependence of the time at which the system
dynamics freezes. Our implementation can be configured in situ to
realize a wide variety of different spin networks, each of which can be
monitored as it moves towards a low-energy configuration. This
programmable artificial spin network bridges the gap between the
theoretical study of ideal isolated spin networks and the experimental
investigation of bulk magnetic samples. Moreover, with an increased
number of spins, such a system may provide a practical physical means to
implement a quantum algorithm, possibly allowing more-effective
approaches to solving certain classes of hard combinatorial optimization
problems.
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