Theory of Cubic-Phase Dynamics in the Linear Potential

Abstract

A quantum wave packet in a linear potential, i.e., under a constant force such as gravity, accumulates a cubic-in-time phase that is universal across Schrodinger-type platforms and naturally realized by Airy eigenstates. Because the classical action is quadratic in the force, this phase comprises exactly three contributions: intrinsic, force-induced, and a cross term. The force-induced contribution alone is shape-independent, whereas the Airy eigenstate renders the shape-dependent contributions non-dispersing. An eigenstate-based nondimensionalization identifies the eigenforce, namely the intrinsic force underlying the packet's acceleration in the absence of an applied force, as a natural parameter. As a function of both forces, the cubic coefficient takes an analytically closed and physically interpretable form that factors along two zero lines: the static Airy eigenstate and a nontrivial zero at which the phase cancels without stationarity. This exposes the eigenforce as an effective antagonist to the applied force, not only in the caustic's self-acceleration but also within the phase, while leaving the centroid unaffected in accordance with Ehrenfest's theorem. Spatially uniform within each packet, the phase cannot be measured directly and is accessible only through the relative phase of two colliding packets, each evolving in its own potential. The general relative cubic coefficient, forbidden by symmetry for identical packets and activated by preparation asymmetry, therefore provides a designable signal. Extracted through heterodyne demodulation of the simulated interference between two Airy packets, its central value agrees with the prediction to sub-percent accuracy within the fitting uncertainty. The analysis spans ultracold-atom condensates, paraxial optics, and surface-gravity water waves.