Exploring Quantum Control Landscape Structure

TL;DR

This paper investigates the structure of quantum control landscapes, demonstrating that they are generally simple and conducive to efficient optimization, which explains the ease of achieving optimal control in quantum systems.

Contribution

It introduces a metric to quantify the linearity of optimization trajectories and provides computational evidence that quantum control landscapes are structurally simple.

Findings

Control trajectories are often nearly straight, indicating simple landscape structure.

Quantum control landscapes tend to be free of complex suboptimal critical points.

The simple landscape structure facilitates efficient optimization of quantum controls.

Abstract

A common goal of quantum control is to maximize a physical observable through the application of a tailored field. The observable value as a function of the field constitutes a quantum control landscape. Previous works have shown, under specified conditions, that the quantum control landscape should be free of suboptimal critical points. This favorable landscape topology is one factor contributing to the efficiency of climbing the landscape. An additional, complementary factor is the landscape \textit{structure}, which constitutes all non-topological features. If the landscape's structure is too complex, then climbs may be forced to take inefficient convoluted routes to finding optimal controls. This paper provides a foundation for understanding control landscape structure by examining the linearity of gradient-based optimization trajectories through the space of control fields. For this assessment, a metric $R\geq 1$ is defined as the ratio of the path length of the optimization trajectory to the Euclidean distance between the initial control field and the resultant optimal control field that takes an observable from the bottom to the top of the landscape. Computational analyses for simple model quantum systems are performed to ascertain the relative abundance of nearly straight control trajectories encountered when optimizing a state-to-state transition probability. The collected results indicate that quantum control landscapes have very simple structural features. The favorable topology and the complementary simple structure of the control landscape provide a basis for understanding the generally observed ease of optimizing a state-to-state transition probability.