Quantumness Witnesses

We introduce a method to witness the quantumness of a system. The method relies on the fact that the anticommutator of two classical states is always positive, while there is always a nonpositive anticommutator due to any two quantum states. We notice that interference depends on the trace of the anticommutator of two states and it is therefore more natural to detect quantumness by looking at anticommutators of states rather than their commutators.