(a) Classical potential |$V(q)$| (thick solid line) and initial Gaussian wave packet |$|\psi(q,0)|^{2}=|\psi_{\rm FG}(q,0)|^{2}$| with |$(q_{c}(0),p_{c}(0))=(0,1.8)$| (thin solid line). The effective potential |$V_{c}(q_{c})$|⁠, Eq. (63), is also plotted as the dashed line. (b) Trajectories in the metastable cubic potential system. The quantum trajectory |$\langle\hat{q}(t)\rangle$|⁠, Nambu trajectory |$x_{1}(t)$|⁠, and classical trajectory |$q(t)$| are given by the dots, solid line, and dashed line, respectively.