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Entanglement witnesses and a loophole problem

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arxiv quant-ph/0608058 v2 pith:6DJOX7BB submitted 2006-08-07 quant-ph

Entanglement witnesses and a loophole problem

classification quant-ph
keywords localentanglementloopholewitnessdecompositiondetectorsexpectationthreshold
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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We consider a possible detector-efficiency loophole in experiments that detect entanglement via the local measurement of witness operators. Here, only local properties of the detectors are known. We derive a general threshold for the detector efficiencies which guarantees that a negative expectation value of a witness is due to entanglement, rather than to erroneous detectors. This threshold depends on the local decomposition of the witness and its measured expectation value. For two-qubit witnesses we find the local operator decomposition that is optimal with respect to closing the loophole.

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