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.
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