Entanglement dynamics in two-qubit open system interacting with a squeezed thermal bath via quantum nondemolition interaction

We analyze the dynamics of entanglement in a two-qubit systeminteracting with an initially squeezed thermal environment via aquantum nondemolition system-reservoir interaction, with the systemand reservoir assumed to be initially separable. We compare andcontrast the decoherence of the two-qubit system in the case wherethe qubits are mutually close-by (`collective regime’) or distant(`localized regime’) with respect to the spatial variation of theenvironment. Sudden death of entanglement (as quantified byconcurrence) is shown to occur in the localized case rather than inthe collective case, where entanglement tends to `ring down’.A consequence of the QND character of the interaction is that thetime-evolved fidelity of a Bell state never falls below \(1/\sqrt{2}\),a fact that is useful for quantum communication applicationslike a quantum repeater. Usinga novel quantification of mixed state entanglement, we show thatthere are noise regimes where even though entanglementvanishes, the state is still available forapplications like NMR quantum computation, because of the presenceof a pseudo-pure component.