# Quantum mechanics and speed limit of ultrafast local control in spin chains

## TL;DR

This paper investigates how to optimize the speed and fidelity of ultrafast local control in spin chains, revealing system-independent features and quantum speed limits that can enhance quantum system manipulation.

## Contribution

It introduces a method to significantly reduce infidelity in ultrafast spin chain transformations by designing optimal control profiles, supported by analytical predictions and numerical validation.

## Key findings

- Optimal control profiles drastically decrease infidelity.
- Qualitative features of control are system-independent.
- Restrictions on control shape persist up to adiabatic limits.

## Abstract

We study optimization of fidelity for ultrafast transformation of a spin chain via external control of a local exchange coupling. We show that infidelity of such a process can be dramatically decreased by choosing a proper control profile in nonadiabatic time domain, predict main features of this profile analytically, corroborate them numerically with a gradient search algorithm, and discuss the corresponding quantum speed limit. For ultrafast transformations, the qualitative features of the obtained optimal control are system-independent. Moreover, the main restrictions on its shape do not depend on the transformation time and remain valid up to the adiabatic limit. Our results can be applied to control a broad variety of quantum systems.