Locally suppressed transverse-field protocol for diabatic quantum annealing

Diabatic quantum annealing (DQA) is an alternative algorithm to adiabatic quantum annealing that can be used to circumvent the exponential slowdown caused by small minima in the annealing energy spectrum. We present the locally suppressed transverse-field (LSTF) protocol, a heuristic method for making stoquastic optimization problems compatible with DQA. We show that, provided an optimization problem intrinsically has magnetic frustration due to inhomogeneous local fields, a target qubit in the problem can always be manipulated to create a double minimum in the energy gap between the ground and first excited states during the evolution of the algorithm. Such a double energy minimum can be exploited to induce diabatic transitions to the first excited state and back to the ground state. In addition to its relevance to classical and quantum algorithmic speedups, the LSTF protocol enables DQA proof-of-principle and physics experiments to be performed on existing hardware, provided independent controls exist for the transverse qubit magnetization fields. We discuss the implications on the coherence requirements of the quantum annealing hardware when using the LSTF protocol, considering specifically the cases of relaxation and dephasing. We show that the relaxation rate of a large system can be made to depend only on the target qubit, presenting opportunities for the characterization of the decohering environment in a quantum annealing processor

Field-enhanced direct tunneling in ultrathin atomic-layer-deposition-grown Au-Al2O3-Cr metal-insulator-metal structures

Metal-insulator-metal structures based on ultrathin high-k dielectric films are underpinning a rapidly increasing number of devices and applications. Here, we report detailed electrical characterizations of asymmetric metal-insulator-metal devices featuring atomic layer deposited 2-nm-thick Al2O3 films. We find a high consistency in the current density as a function of applied electric field between devices with very different surface areas and significant asymmetries in the IV characteristics. We show by TEM that the thickness of the dielectric film and the quality of the metal-insulator interfaces are highly uniform and of high quality, respectively. In addition, we develop a model which accounts for the field enhancement due to the small sharp features on the electrode surface and show that this can very accurately describe the observed asymmetry in the current-voltage characteristic, which cannot be explained by the difference in work function alone