Bagan E, Ballester M A, Muñoz-Tapia R, Romero-Isart O
Grup de Física Teòrica and IFAE, Facultat de Ciències, Edifici Cn, Universitat Autònoma de Barcelona, 08193 Bellaterra, Barcelona, Spain.
Phys Rev Lett. 2005 Sep 9;95(11):110504. doi: 10.1103/PhysRevLett.95.110504.
Given a large number N of copies of a qubit state of which we wish to estimate its purity, we prove that separable-measurement protocols can be as efficient as the optimal joint-measurement one if classical communication is used. This shows that the optimal estimation of the entanglement of a two-qubit state can also be achieved asymptotically with fully separable measurements. Thus, quantum memories provide no advantage in this situation. The relationship between our global Bayesian approach and the quantum Cramér-Rao bound is discussed.
给定大量N个量子比特态的副本，我们希望估计其纯度。我们证明，如果使用经典通信，可分测量协议可以与最优联合测量协议一样高效。这表明，两量子比特态纠缠的最优估计也可以通过完全可分测量渐近地实现。因此，在这种情况下量子存储器没有优势。我们还讨论了全局贝叶斯方法与量子克拉美-罗界之间的关系。
