01086nas a2200181 4500008004100000022001400041245005000055210005000105260003900155300001100194490000700205520055000212100001700762700001900779700002400798700002400822856005800846 2005 eng d a0031-900700aPurity Estimation with Separable Measurements0 aPurity Estimation with Separable Measurements bAmerican Physical Societyc09/2005 a1105040 v953 aGiven 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.1 aBagan, Emili1 aBallester, M A1 aMuñoz-Tapia, Ramon1 aRomero-Isart, Oriol uhttp://link.aps.org/doi/10.1103/PhysRevLett.95.110504