Physics – Quantum Physics
Scientific paper
2005-05-11
Enlarged version of Phys. Rev. Lett. 95, 110504 (2005)
Physics
Quantum Physics
5 pages, 1 figure, RevTeX, improved version
Scientific paper
10.1103/PhysRevLett.95.110504
Given a finite number $N$ of copies of a qubit state we compute the maximum fidelity that can be attained using joint-measurement protocols for estimating its purity. We prove that in the asymptotic $N\to\infty$ limit, separable-measurement protocols can be as efficient as the optimal joint-measurement one if classical communication is used. This in turn shows that the optimal estimation of the entanglement of a two-qubit state can also be achieved asymptotically with fully separable measurements. The relationship between our global Bayesian approach and the quantum Cramer-Rao bound is also discussed.
Bagan Emili
Ballester Manuel A.
Munoz-Tapia Ramon
Romero-Isart Oriol
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