Optimal noise-canceling shortcuts to adiabaticity: application to noisy Majorana-based gates

TL;DR

This paper develops noise-optimized protocols for Majorana-based quantum gates that counteract high-frequency noise effects, improving gate fidelity by tailoring control sequences to known noise levels.

Contribution

It introduces a method to design optimal control protocols for Majorana gates that specifically account for and mitigate the effects of unavoidable noise, using simulated annealing and Pontryagin's principle.

Findings

Noise-optimized protocols eliminate antiadiabatic behavior.

Protocols are bang-bang for zero noise and continuous with noise.

Optimized protocols significantly improve gate robustness.

Abstract

Adiabatic braiding of Majorana zero modes can be used for topologically protected quantum information processing. While extremely robust to many environmental perturbations, these systems are vulnerable to noise with high-frequency components. Ironically, slower processes needed for adiabaticity allow more noise-induced excitations to accumulate, resulting in an antiadiabatic behavior that limits the precision of Majorana gates if some noise is present. In a recent publication [Phys. Rev. B 96, 075158 (2017)], fast optimal protocols were proposed as a shortcut for implementing the same unitary operation as the adiabatic braiding. These shortcuts sacrifice topological protection in the absence of noise but provide performance gains and remarkable robustness to noise due to the shorter evolution time. Nevertheless, gates optimized for vanishing noise are suboptimal in the presence of noise. If we know the noise strength beforehand, can we design protocols optimized for the existing unavoidable noise, which will effectively correct the noise-induced errors? We address this question in the present paper. We find such optimal protocols using simulated-annealing Monte Carlo simulations. The numerically found pulse shapes, which we fully characterize, are in agreement with Pontryagin's minimum principle, which allows us to arbitrarily improve the approximate numerical results (due to discretization and imperfect convergence) and obtain numerically exact optimal protocols. The protocols are \textit{bang-bang} (sequence of sudden quenches) for vanishing noise, but contain continuous segments in the presence of multiplicative noise due to the nonlinearity of the master equation governing the evolution. We find that such noise-optimized protocols completely eliminate the above-mentioned antiadiabatic behavior.