Quasi-equilibrium and quantum correlation in an open spin-pair system

Abstract

Quasi-equilibrium states that can be prepared in solids through Nuclear Magnetic Resonance (NMR) techniques are out-of-equilibrium states that slowly relax towards thermodynamic equilibrium with the lattice. In this work, we use the quantum discord dynamics as a witness of the quantum correlation in this kind of state. The studied system is a set of dipole interacting spin pairs whose initial state is prepared with the NMR Jeener–Broekaert pulse sequence, starting from equilibrium at high temperature and high external magnetic field. It then evolves as an open quantum system within two consecutive dynamic scenarios: adiabatic decoherence driven by the coupling of the pairs to a common phonon field, described within a non-Markovian approach, and spin–lattice relaxation represented by the high-temperature limit of the Born–Markov master equation, and driven by thermal fluctuations. In this way, the studied model is endowed with the dynamics of a realistic solid sample. The quantum discord rapidly increases during the preparation of the initial state, escalating several orders of magnitude compared with thermal equilibrium at room temperature. During decoherence—despite the decay of coherences—the quantum discord oscillates upon this high value, holding the same value as the initial state. Finally, the quantum discord dissipates within a time scale shorter than but comparable to spin–lattice relaxation.