Limitation of entanglement due to spatial qubit separation

Institut für Physik, Universität Augsburg - Universitätsstraße 1 D-86135 Augsburg, Germany

Corresponding author: roland.doll@physik.uni-augsburg.de

Received: 23 August 2006
Accepted: 27 September 2006

Abstract

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.