Figure 4

We plot the nonlinearity measure \(\pmb{\eta_{ { \mathrm{N G}} } [{V} ]}\) against the normalized measure of \(\pmb{\nu(\psi_{0})}\) nonclassicality based on the negative parts of the Wigner-function for the GS of the undriven Duffing resonator. We show that the two measures are monotone functions of each other. The squares show the values of \(\eta_{\mathrm{NG}}\) at set nonclassicality of the ground state of the undriven Duffing oscillator. The solid line represents the fit given by Eq. (11). After an initial trait where \(\eta_{\mathrm{NG}}\) grows as \(\sqrt{\nu(\psi_{0})}\), the relation between the two figures of merit becomes approximately linear, showing the direct connection between nonlinearity and nonclassical character of the state of the oscillator.