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physica status solidi (b)

Entanglement swapping in presence of dephasing

Jerzy Dajka, J. Łuczka, Peter Hänggi A

Published Apr 3, 2009• 2 citations
Publisher

Abstract

Abstract Entanglement which originates from a pair of non‐interacting qubits by means of entanglement swapping is discussed when dephasing mechanisms are present. Then, the exact reduced dynamics can be evaluated for two pairs of independent qubits which each are coupled to their own bosonic environment. Each of two pairs of qubits is initially maximally entangled. The conditions for the entanglement survival at asymptotic long times in terms of environment spectral properties are elaborated. It is shown that these conditions hold true also for the inter–pair entanglement stemming. Entanglement swapping can effectively been controlled by a proper choice of initial state of the finite environment (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

DOI
10.1002/pssb.200881551

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Abstract Entanglement which originates from a pair of non‐interacting qubits by means of entanglement swapping is discussed when dephasing mechanisms are present. Then, the exact reduced dynamics can be evaluated for two pairs of independent qubits which each are coupled to their own bosonic environment. Each of two pairs of qubits is initially maximally entangled. The conditions for the entanglement survival at asymptotic long times in terms of environment spectral properties are elaborated. It is shown that these conditions hold true also for the inter–pair entanglement stemming. Entanglement swapping can effectively been controlled by a proper choice of initial state of the finite environment (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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Abstract We study the entanglement creation between two flux qubits interacting with electromagnetic field modes. No direct interaction between the qubits exists. Entanglement is reached using the entanglement swapping method by an interference measurement performed on photons. We discuss the influence of off‐resonance and multi‐photon initial states on the qubit–qubit entanglement. The presented scheme is able to drive an initially separable state of two qubits into an highly entangled state suitable for quantum information processing (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
The evolution of quantum entanglement of a pair of non-interacting qubits is studied for the case where one of them is non-dissipatively but dephasingly coupled to the environment. The reduced non-Markovian dynamics of the qubits is exact for an arbitrary strength of coupling to the environment and the arbitrary frequency spectrum of environment fluctuations. While for the subohmic and ohmic environments the entanglement diminishes, for the superohmic zero-temperature environment it survives for a long time.
Abstract The problem of decoherence in recently proposed flux qubits built on mesoscopic normal metal rings is studied. The relaxation and dephasing times coming from the interaction of the qubit with the measuring device are calculated within the weak coupling spin‐boson model. We analyze the effect of the measuring apparatus on the relaxation and dephasing times. The possibility of ohmic approximation for the bath is investigated. Considerably long decoherence times are found. The work is limited to the weak damping regime in low temperatures. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
Decoherence of qubits is one main limitation in quantum information processing, where the qubits themselves can be mixed because of decoherence and additional decoherence may generated through interactions with a bath. Here, we present a complete study of qubit-environment entanglement in the case of interactions that lead to pure dephasing of the qubit by allowing the initial state of the qubit to be mixed, exceeding previous restrictions. We find that the criteria for separability that hold for pure initial states of the qubit are reduced to witness status. Yet, the evolution of the witnesses reflects the array of features that entanglement can exhibit in the mixed case but are forbidden in the pure-qubit case. Most notably, this includes sudden birth of entanglement as the initial phase of the evolution is typically separable. Moreover, there is a stark numerical advantage to the study of entanglement for large environments when using the witness rather than the traditional measures available. We include the study of entanglement generated between a spin qubit and a sparse spin environment to illustrate the peculiarities of the evolution of entanglement in this case, as well as to emphasize the level of reliability of the entanglement witness.
In this paper, we investigate the possibility of entanglement swapping between two distinct qubits coupled to their own (in general) non-Markovian environments. This goal is achieved by performing Bell state measurement on photons leaving dissipative cavities. In the continuation, we introduce the concept of entangling power to measure the average of swapped entanglement over all possible pure initial states. Then we present our results in two regimes, strong and weak coupling, and discuss the role of the detuning parameter in each regime in the amount of swapped entanglement. We also determine the conditions under which the maximum amount of entanglement can be swapped between the two qubits. It is revealed that, despite the presence of dissipation, it is possible to create long-living stationary entanglement between the two qubits.