Advantages of Randomization in Coherent Quantum Dynamical Control

TL;DR

This paper reviews and demonstrates how randomized control strategies can significantly improve the suppression of unwanted interactions in closed quantum systems, especially in spin chains, by combining randomization with deterministic methods.

Contribution

It introduces and analyzes efficient randomized decoupling schemes that outperform traditional methods, including explicit constructions removing interactions up to third order.

Findings

Randomization enhances long-term quantum control performance.

Efficient decoupling schemes scale better with system size.

Sequences removing interactions up to third order are explicitly constructed.

Abstract

Control scenarios have been identified where the use of randomized design may substantially improve the performance of dynamical decoupling methods [L. F. Santos and L. Viola, Phys. Rev. Lett. {\bf 97}, 150501 (2006)]. Here, by focusing on the suppression of internal unwanted interactions in closed quantum systems, we review and further elaborate on the advantages of randomization at long evolution times. By way of illustration, special emphasis is devoted to isolated Heisenberg-coupled chains of spin-1/2 particles. In particular, for nearest-neighbor interactions, two types of decoupling cycles are contrasted: inefficient averaging, whereby the number of control actions increases exponentially with the system size, and efficient averaging associated to a fixed-size control group. The latter allows for analytical and numerical studies of efficient decoupling schemes created by exploiting and merging together randomization and deterministic strategies, such as symmetrization, concatenation, and cyclic permutations. Notably, sequences capable to remove interactions up to third order are explicitly constructed. The consequences of faulty controls are also analyzed.