Entanglement Dynamics in Two-Qubit Open System Interacting with a Squeezed Thermal Bath via Quantum Nondemolition interaction

TL;DR

This paper investigates how entanglement in a two-qubit system evolves when interacting with a squeezed thermal bath via quantum nondemolition coupling, revealing conditions for entanglement sudden death and implications for quantum communication.

Contribution

It provides a detailed analysis of entanglement dynamics in QND interactions with a squeezed thermal environment, highlighting differences between collective and localized regimes.

Findings

Entanglement experiences sudden death in localized regimes.

Fidelity of Bell states remains above 1/√2 due to QND interaction.

Noise regimes exist where entanglement vanishes but states remain useful for quantum computation.

Abstract

We analyze the dynamics of entanglement in a two-qubit system interacting with an initially squeezed thermal environment via a quantum nondemolition system-reservoir interaction, with the system and reservoir assumed to be initially separable. We compare and contrast the decoherence of the two-qubit system in the case where the qubits are mutually close-by (`collective regime') or distant (`localized regime') with respect to the spatial variation of the environment. Sudden death of entanglement (as quantified by concurrence) is shown to occur in the localized case rather than in the collective case, where entanglement tends to `ring down'. A consequence of the QND character of the interaction is that the time-evolved fidelity of a Bell state never falls below $1/\sqrt{2}$, a fact that is useful for quantum communication applications like a quantum repeater. Using a novel quantification of mixed state entanglement, we show that there are noise regimes where even though entanglement vanishes, the state is still available for applications like NMR quantum computation, because of the presence of a pseudo-pure component.