Universal observable detecting all two-qubit entanglement and determinant-based separability tests
Universal observable detecting all two-qubit entanglement and determinant-based separability tests
We construct a single observable measurement of which mean value on four copies of an {\it unknown} two-qubit state is sufficient for unambiguous decision whether the state is separable or entangled. In other words, there exists a universal collective entanglement witness detecting all two-qubit entanglement. The test is directly linked …