Experimental Direct Observation of Mixed State Entanglement
Christian Schmid, Nikolai Kiesel, Witlef Wieczorek, Harald Weinfurter, Florian Mintert, Andreas Buchleitner · Physical Review Letters · 2008
We report on the direct estimation of concurrence for mixed quantum states. The used method relies on joint measurements on two copies of an entangled state. In the experimental demonstration two polarization-entangled photon pairs emitted from spontaneous parametric down-conversion are analyzed together using a linear optics controlled phase gate. We demonstrate that the measured data, without need for further numerical processing, directly yield reliable estimates, despite experimental imperfections.