Nature440, 1022-1024 (20 April 2006) | doi:10.1038/nature04627; Received 6 December 2005; ; Accepted 27 January 2006
Experimental determination of entanglement with a single measurement
S. P. Walborn1, P. H. Souto Ribeiro1, L. Davidovich1, F. Mintert1,2 and A. Buchleitner3
Nearly all protocols requiring shared quantum information1—such as quantum teleportation2 or key distribution3—rely on entanglement between distant parties. However, entanglement is difficult to characterize experimentally. All existing techniques for doing so, including entanglement witnesses4, 11, 12 or Bell inequalities5, disclose the entanglement of some quantum states but fail for other states; therefore, they cannot provide satisfactory results in general. Such methods are fundamentally different from entanglement measures that, by definition, quantify the amount of entanglement in any state. However, these measures suffer from the severe disadvantage that they typically are not directly accessible in laboratory experiments. Here we report a linear optics experiment in which we directly observe a pure-state entanglement measure, namely concurrence6. Our measurement set-up includes two copies of a quantum state: these 'twin' states are prepared in the polarization and momentum degrees of freedom of two photons, and concurrence is measured with a single, local measurement on just one of the photons.
Instituto de Física, Universidade Federal do Rio de Janeiro, Caixa Postal 68528, Rio de Janeiro, RJ 21941-972, Brazil
Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA