Scaling and diabatic effects in quantum annealing with a D-Wave device

We discuss quantum annealing of the two-dimensional transverse-field Ising model implemented on a D-Wave device in the form of $L \times L$ lattices with $L \le 32$. Analyzing the residual energy and deviation from maximal magnetization in the final classical state, we find an optimal $L$ dependent annealing rate $v$ for which the two quantities are minimized. The results are well described by a phenomenological model with two powers of $v$ and $L$-dependent prefactors to describe the competing effects of reduced quantum fluctuations and increasing noise impact when $v$ is lowered. The same scaling form also describes results of numerical solutions of a transverse-field Ising chain with the spins coupled to noise sources. We explain why the optimal annealing time is much longer than the coherence time of the individual qubits.