Abstract：
:Nearly all protocols requiring shared quantum information--such as quantum teleportation or key distribution--rely on entanglement between distant parties. However, entanglement is difficult to characterize experimentally. All existing techniques for doing so, including entanglement witnesses or Bell inequalities, 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 concurrence. 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.
journal_name
Naturejournal_title
Natureauthors
Walborn SP,Souto Ribeiro PH,Davidovich L,Mintert F,Buchleitner Adoi
10.1038/nature04627subject
Has Abstractpub_date
2006-04-20 00:00:00pages
1022-4issue
7087eissn
0028-0836issn
1476-4687pii
nature04627journal_volume
440pub_type
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