Quantum mechanics of ultrafast local control in spin chains

P. V. Pyshkin, Eugene Ya. Sherman, Lian-Ao Wu

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.

Joachim Stolze, Taras Verkholyak, Oleg Derzhko +1 more

Navin Khaneja, Steffen J. Glaser

Experiments in coherent spectroscopy correspond to control of quantum mechanical ensembles guiding them from initial to final target states by unitary transformations. The control inputs (pulse sequences) that accomplish these unitary transformations should take as little time as possible so as to minimize the effects of relaxation and to optimize the sensitivity of the experiments. Here, we present a radically different and generally applicable approach to efficient control of dynamics in spin chains of arbitrary length. The approach relies on the creation of localized spin waves, ``spin solitons'', and efficient propagation of these soliton states through the spin chain. The methods presented are expected to find immediate applications in control of spin dynamics in coherent spectroscopy and quantum information processing.

J.G. Carvalho, Djalma Medeiros, G. G. Cabrera

Kristel F. L. Michielsen

Y. del Castillo, Alejandro Ferrón, Joaquín Fernández-Rossier

We consider the remote manipulation of the quantum state of the edge emergent spins of Haldane spin chains using a weak local perturbation on the opposite edge. We derive an effective four-level model that correctly captures the response of the local magnetization to local perturbations and we use it to show that applying a small local field on one edge of the chain induces a strong variation of the magnetization on the opposite edge. Using a Landau-Zener protocol, we show how local control of the field on one edge of the chain, implemented, for instance, with a spin-polarized scanning tunneling microscope tip, can adiabatically switch the magnetization direction on the other side of the chain.