Unifying entanglement and nonclassicality via the Narcowich-Wigner spectrum

Entanglement and nonclassicality are two fundamental features of quantum mechanics which serve as resources in quantum information processing tasks and are tightly connected by two-mode mixing. In this work, we show that the connection between entanglement and nonclassicality extends well beyond the two-mode mixing setting by considering the case of arbitrary multimode Gaussian evolutions. Using the Narcowich-Wigner spectrum as our main technical tool to quantify single-mode phase-space nonclassicality, we derive sufficient conditions for a single-mode state to generate NPT entanglement after a multimode Gaussian unitary evolution, which both tighten and generalize existing criteria. As an application, we obtain simple single-mode protocols for quantitatively testing the entanglement structure of multimode quantum states over arbitrary bipartitions. Our results identify the Narcowich-Wigner spectrum as a powerful tool for investigating fundamental properties of bosonic quantum states.