  We consider spatially separated qubits coupled to a thermal bosonic field
that causes pure dephasing. Our focus is on the entanglement of two Bell states
which for vanishing separation are known as robust and fragile entangled
states. The reduced two-qubit dynamics is solved exactly and explicitly. Our
results allow us to gain information about the robustness of two-qubit
decoherence-free subspaces with respect to physical parameters such as
temperature, qubit-bath coupling strength and spatial separation of the qubits.
Moreover, we clarify the relation between single-qubit coherence and two-qubit
entanglement and identify parameter regimes in which the terms robust and
fragile are no longer appropriate.
