Closed-Loop Designed Open-Loop Control of Quantum Systems: An Error Analysis

TL;DR

This paper analyzes the error in closed-loop designed open-loop control of quantum systems, showing how simulation accuracy affects convergence and providing bounds on the error norm.

Contribution

It provides a theoretical analysis of the error dynamics in closed-loop designed open-loop quantum control, including convergence properties and bounds.

Findings

Error converges to a unitary transformation of initial error

Increasing simulation accuracy beyond a threshold does not significantly improve convergence

An upper bound on the error norm is established

Abstract

Quantum Lyapunov control, an important class of quantum control methods, aims at generating converging dynamics guided by Lyapunov-based theoretical tools. However, unlike the case of classical systems, disturbance caused by quantum measurement hinders direct and exact realization of the theoretical feedback dynamics designed with Lyapunov theory. Regarding this issue, the idea of closed-loop designed open-loop control has been mentioned in literature, which means to design the closed-loop dynamics theoretically, simulate the closed-loop system, generate control pulses based on simulation and apply them to the real plant in an open-loop fashion. Based on bilinear quantum control model, we analyze in this article the error, i.e., difference between the theoretical and real systems' time-evolved states, incurred by the procedures of closed-loop designed open-loop control. It is proved that the error at an arbitrary time converges to a unitary transformation of initialization error as the number of simulation grids between 0 and that time tends to infinity. Moreover, it is found that once the simulation accuracy reaches a certain level, adopting more accurate (thus possibly more expensive) numerical simulation methods does not efficiently improve convergence. We also present an upper bound on the error norm and an example to illustrate our results.