Geometric representation of two-qubit entanglement witnesses

@article{Milne2014GeometricRO,
  title={Geometric representation of two-qubit entanglement witnesses},
  author={Antony Milne and David Jennings and Terry Rudolph},
  journal={Physical Review A},
  year={2014},
  volume={92}
}
Any two-qubit state can be represented geometrically by a steering ellipsoid inside the Blochsphere. We extend this approach to represent any block positive two-qubit operator B. We derive a classification scheme based on the positivity of detBand de t B T B; this shows that any ellipsoid inside the Bloch sphere must represent either a two-qubit state or a two-qubit entanglement witness. We focus on such witnesses and their corresponding ellipsoids, finding that properties such as witness… 
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