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Experimental determination of entanglement with a single measurement


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.

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Figure 1: Experimental set-up for the measurement of entanglement using two copies of the quantum state.
Figure 2: Count rates.
Figure 3: Experimental values of concurrence.


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We acknowledge support from the Brazilian agencies CNPq, CAPES, PRONEX, FUJB and FAPERJ. This work was performed as part of the Brazilian Millennium Institute for Quantum Information, and was supported by a fellowship within the postdoctoral programme of the German Academic Exchange Service (DAAD) as well as by a Feodor Lynen fellowship of the Alexander von Humboldt foundation. We are indebted to C. H. Monken for discussions.

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Correspondence to S. P. Walborn.

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Walborn, S., Souto Ribeiro, P., Davidovich, L. et al. Experimental determination of entanglement with a single measurement. Nature 440, 1022–1024 (2006).

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