Two-qubit quantum correlations versus single-qubit population

It is considered a system made by two noninteracting qubits, initially entangled, embedded in zero-temperature bosonic independent environments. It is shown that different forms of quantum correlations for two qubits can be expressed in terms of excited state population of each single qubit. These relations are explicitly given for both entanglement and Bell function. This permits to evidence regions where there is entanglement without violation of a Bell inequality, then showing that entanglement does not necessarily witness the presence of quantum correlations nonreproducible by a classical local model, as identified by Bell inequality violations. We finally report the explicit time dependence connection between quantum correlations dynamics and spectral densities of the environments. To this purpose we shall examine systems where non-Markovian features are relevant as in the case of cavity QED systems and of structured environments like photonic crystals.