Decoherence dynamics of a qubit coupled to a quantum two-level system
  We study the decoherence dynamics of a qubit coupled to a quantum two-level system (TLS) in addition to its weak coupling to a background environment. We analyze the different regimes of behaviour that arise as the values of the different parameters are varied. We classify those regimes as two weak-coupling regimes, which differ by the relation between the qubit and TLS decoherence times, and a strong-coupling one. We also find analytic expressions describing the decoherence rates in the weak-coupling regimes, and we verify numerically that those expressions have a rather wide range of validity. Along with obtaining the above-mentioned results, we address the questions of qubit-TLS entanglement and the additivity of multiple TLS contributions. We also discuss the transition from weak to strong coupling as the parameters are varied, and we numerically determine the location of the boundary between the two regimes. 
S. Ashhab, J. R. Johansson, and Franco Nori