  To quantify single mode nonclassicality, we start from an operational
approach. A positive semi-definite observable is introduced to describe a
measurement setup. The quantification is based on the negativity of the
normally ordered version of this observable. Perfect operational quantumness
corresponds to the quantum-noise-free measurement of the chosen observable.
Surprisingly, even moderately squeezed states may exhibit perfect quantumness
for a properly designed measurement. The quantification is also considered from
an axiomatic viewpoint, based on the algebraic structure of the quantum states
and the quantum superposition principle. Basic conclusions from both approaches
are consistent with this fundamental principle of the quantum world.
