The extended Bloch Representation of Entanglement and Measurement in   Quantum Mechanics
  The quantum formalism can be completed by assuming that a density operator can also represent a pure state. An 'extended Bloch representation' (EBR) then results, in which not only states, but also the measurement-interactions can be represented. The Born rule is obtained as an expression of the subjective lack of knowledge about the measurement-interaction that is each time actualized. Entanglement can also be consistently described in the EBR, as it remains compatible with the principle according to which a composite entity exists only if its components also exist, and therefore are in pure states. 
Diederik Aerts, Massimiliano Sassoli de Bianchi and Sandro Sozzo
quant-ph