Unitary invariants and classification of four-qubit states via negativity fonts
S. Shelly Sharma, Naresh Kumar Sharma · Physical Review A · 2013
Local unitary invariance and the notion of negativity fonts are used as the principal tools to construct four-qubit invariants of degree 8, 12, and 24. A degree-8 polynomial invariant that is nonzero on pure four-qubit states with four-body quantum correlations and zero on all other states is identified. Classification of four-qubit states into seven major classes, using criteria based on the nature of correlations, is discussed.