Anneal-path correction in flux qubits

Abstract

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.

Locally suppressed transverse-field protocol for diabatic quantum annealing

“…The need for increased flexibility of the annealing schedules, including independent control of the transverse fields, has been widely recognized (see Ref. [33] and references therein), putting such capabilities on the roadmap for future devices [29,31,34,44,45].…”

“…To this end, we first introduce a simple model of magnetic frustration to show that quantum tunneling effects can be leveraged to create a small avoided level crossing. We then introduce a means of creating an additional avoided crossing via customized annealing schedules [32][33][34] that allows us to sketch a heuristic algorithm for finding lower-energy eigenstates of the problem than in AQA under certain circumstances. We also provide a statistical analysis of randomly generated examples to demonstrate the performance of LSTF-DQA and we analyze in detail a single instance to discuss the consequences of its application.…”

Demonstration of long-range correlations via susceptibility measurements in a one-dimensional superconducting Josephson spin chain

“…This single unit method of identifying both the qubit and coupler's spin components is not as universally applicable as more general means such as the Schrieffer-Wolff transformation [60], particularly in the strong coupling regime. However, as we will restrict our analysis to the weak coupling limit, the results of the two methods coincide [16,[61][62][63]. With these assumptions, the behavior of the physical device can be mapped to the one-dimensional, transverse field Ising spin model.…”

Locally Suppressed Transverse-Field Protocol for Diabatic Quantum Annealing

“…Finally we note that commercial annealers do not yet offer the ability to control the transverse fields of the qubits independently [44]. The need for increased flexibility of the annealing schedules, including independent control of the transverse fields, has been widely recognized (see [33] and references therein), putting such capabilities on the roadmap for future devices [29,31,34,45,46].…”

“…To this end, we first introduce a simple model of magnetic frustration to show that quantum tunneling effects can be leveraged to create a small avoided level crossing. We then introduce a means of creating an additional avoided crossing via customized annealing schedules [32][33][34] that allows us to sketch a heuristic algorithm for finding lower energy eigenstates of the problem than in AQA under certain circumstances. We also provide a randomly generated example to demonstrate LSTF-DQA and discuss the consequences of its application.…”