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Entropic quantifier of spin–boson nonclassicality

Spin systems interacting with boson environments are ubiquitous in nature. To what extent quantum states in such systems depart from classicality is becoming an increasingly important issue. Here, we study nonclassicality in systems involving the interaction between a spin and a boson mode. We introduce a simple and effective method for quantifying spin–boson nonclassicality in terms of the entropy excess between the classical and the quantum Tsallis entropy. The method can be naturally extended to more general systems. Fundamental properties of the nonclassicality quantifier are revealed, which render it reasonable for spin–boson states. Basic features are illustrated by a variety of typical spin–boson states. As applications, we further investigate spin–boson nonclassicality in the Dicke model, which sheds light on the normal-superradiant phase transition and quantum metrology.