Francisco Perez-Bernal

arXiv2019

Excited-state quantum phase transition and the quantum speed limit

We investigate the influence of an excited-state quantum phase transition (ESQPT) on the quantum speed limit (QSL) time of an open quantum system. The system consists of a central qubit coupled to a spin environment modeled by a Lipkin-Meshkov-Glick (LMG) Hamiltonian. We show that when the coupling between qubit and environment is such that the latter is located at the ESQPT critical energy the qubit evolution shows a remarkable slowdown, marked by an abrupt peak in the QSL time. Interestingly, this slowdown at the ESQPT critical point is induced by the singular behavior of the qubit decoherence rate, being independent of the Markovian nature of the environment.

quant-ph↗

arXiv2016

Algebraic theory of endohedrally confined diatomic molecules: application to H$_2$@C$_{60}$

A simple and yet powerful approach for modeling the structure of endohedrally confined diatomic molecules is introduced. The theory, based on a u(4)+u(3) dynamical algebra, combines u(4), the vibron model dynamical algebra, with a u(3) dynamical algebra that models a spherically symmetric three dimensional potential. The first algebra encompasses the internal roto-vibrations degrees of freedom of the molecule, while the second takes into account the confined molecule center-of-mass degrees of freedom. A resulting subalgebra chain is connected to the underlying physics and the model is applied to the prototypical case of H2 caged in a fullerene molecule. The spectrum of the supramolecular complex H2@C60 is described with a few parameters and predictions for not yet detected levels are made. Our fits suggest that the quantum numbers of a few lines should be reassigned to obtain better agreement with data.

physics.chem-ph↗