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Entanglement witnesses and a loophole problem

Patrick Skwara·Hermann Kampermann·Matthias Kleinmann·Dagmar Bruss
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Abstract

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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Patrick Skwara, Hermann Kampermann, Matthias Kleinmann, Dagmar Bruss. 2007-07-18. Entanglement witnesses and a loophole problem. https://doi.org/10.1103/physreva.76.012312

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