  The concept of physical twin observables (PTO) for bipartite quantum
states,introduced and proved relevant for quantum information theory in recent
work, is substantially simplified. The relation of observable and state is
studied in detail from the point of view of coherence entropy. Properties of
this quantity are further explored. It is shown that, besides for pure states,
quantum discord (measure of entanglement) can be expressed through the
coherence entropy of a PTO complete in relation to the state.
