Navigating the noise-depth tradeoff in adiabatic quantum circuits

Adiabatic quantum algorithms solve computational problems by slowly evolving a trivial state to the desired solution. On an ideal quantum computer, the solution quality improves monotonically with increasing circuit depth. By contrast, increasing the depth in current noisy computers introduces more noise and eventually deteriorates any computational advantage. What is the optimal circuit depth that provides the best solution? Here, we address this question by investigating an adiabatic circuit that interpolates between the paramagnetic and ferromagnetic ground states of the one-dimensional quantum Ising model. We characterize the quality of the final output by the density of defects d, as a function of the circuit depth N and noise strength σ. We find that d is well-described by the simple form d_ideal + d_noise, where the ideal case d_ideal ∼ N^-1/2 is controlled by the Kibble-Zurek mechanism, and the noise contribution scales as d_noise ∼ Nσ². It follows that the optimal number of steps minimizing the number of defects goes as ∼ σ^-4/3. We implement this algorithm on a noisy superconducting quantum processor and find that the dependence of the density of defects on the circuit depth follows the predicted non-monotonous behavior and agrees well with noisy simulations. Our work allows one to efficiently benchmark quantum devices and extract their effective noise strength σ.