Creation of long-lived states in interacting spins coupled to a thermal bath

Abstract

We study the classic problem concerning the dissipative dynamics of a system of two interacting spin-1/2 particles, coupled to a thermal bath. In particular, we consider the case of a resonant excitation of the initial single-spin coherences and examine the consequences of the interplay between this excitation and the spin-spin dipolar coupling. Using a fluctuation-regulated Quantum Master Equation (frQME) [Phys. Rev. A 97, 063837 (2018)] we show that the relevant dissipator consists of cross-terms between the drive and the dipolar Hamiltonian, apart from regular self-terms responsible for ordinary relaxation effects. The drive-dipole cross-correlations provide for a novel second-order coupling of single and two-spin observables in the Zeeman basis, which cannot be classified as a dissipative effect. We show that the presence of these unique second-order non-dissipative terms lead to the well-known spin-locked steady-states in dipolar spin-ensembles.Importantly, the mathematical form of the steady-state magnetization in the spin-locked phase, obtained from our theory, matches with the prediction of the traditional spin-temperature based methods used to describe such phenomena.