Quantumness Witnesses

A recently proposed test of quantumness [R. Alicki and N. Van Ryn, J. Phys. A: Math. Theor. 41 062001 (2008)] is put into a broader mathematical and physical perspective. The notion of quantumness witness is introduced, in analogy to entanglement witness, and illustrated by examples of a single qubit and many-body systems with additive observables. We compare also our proposal with the quantumness test based on quantum correlations (entanglement) and Bell inequalities. A class of quantumness witnesses associated to the phase space representation is also discussed.