Task-dependent control of open quantum systems

TL;DR

This paper presents a variational optimization method for controlling open quantum systems to achieve specific tasks, such as state fidelity or entanglement, by designing time-dependent Hamiltonians that account for non-Markovian effects.

Contribution

It introduces a general strategy to optimize system Hamiltonians for desired quantum tasks, including non-unitary effects, considering fast control and non-Markovian dynamics.

Findings

Optimizes quantum tasks via variational Hamiltonian design.

Handles non-Markovian dynamics for improved control.

Can protect against or utilize bath-induced decoherence.

Abstract

We develop a general optimization strategy for performing a chosen unitary or non-unitary task on an open quantum system. The goal is to design a controlled time-dependent system Hamiltonian by variationally minimizing or maximizing a chosen function of the system state, which quantifies the task success (score), such as fidelity, purity, or entanglement. If the time-dependence of the system Hamiltonian is fast enough to be comparable or shorter than the response-time of the bath, then the resulting non-Markovian dynamics is shown to optimize the chosen task score to second order in the coupling to the bath. This strategy can protect a desired unitary system evolution from bath-induced decoherence, but ca also take advantage of the system-bath coupling so as to realize a desired non-unitary effect on the system.