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.