Local vs. joint measurements for the entanglement of assistance

Authors:
T. Laustsen;F. Verstraete;S. J. Van Enk
Affiliations:
Bell Labs, Lucent Technologies, Murray Hill, NJ and Danish Quantum Optics Center, Department of Physics and Astronomy, University of Aarhus, Århus C, Denmark;Bell Labs, Lucent Technologies, Murray Hill, NJ and Department of Mathematical Physics and Astronomy, Ghent University, Gent, Belgium;Bell Labs, Lucent Technologies, Murray Hill, NJ
Venue:
Quantum Information & Computation
Year:
2003

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Abstract

We consider a variant of the entanglement of assistance, as independently introduced by D.P. DiVincenzo et al. (quant-ph/9803033) and O. Cohen (Phys. Rev. Lett. 80, 2493 (1998)). Instead of considering three-party states in which one of the parties, the assistant, performs a measurement such that the remaining two parties are left with on average as much entanglement as possible, we consider four-party states where two parties play the role of assistants. We answer several questions that arise naturally in this scenario, such as (i) how much more entanglement can be produced when the assistants are allowed to perform joint measurements, (ii) for what type of states are local measurements sufficient, (iii) is it necessary for the second assistant to know the measurement outcome of the first, and (iv) are projective measurements sufficient or are more general POVMs needed?