The neutron returning time in a linear potential

TL;DR

This paper calculates quantum time delays for neutrons scattering off Earth's linear gravitational potential, revealing how different definitions relate to classical times and highlighting the role of barrier shape and interference effects.

Contribution

It introduces a new definition of Larmor time aligned with neutron propagation and analyzes its relation to classical returning time in a linear gravitational potential.

Findings

Larmor time coincides with classical returning time in zero magnetic field.

Dwell time matches classical returning time in the classical forbidden region.

Time delays oscillate with de Broglie wavelength, indicating self-interference effects.

Abstract

In this paper, we calculate the quantum time delays for neutron scattering off the Earth's linear gravitational potential. The quantum time delays are obtained by subtracting the classical returning time (CRT) from the Wigner time, the dwell time and the redefined Larmor time respectively. Different from the conventional definition, our Larmor time is defined by aligning the magnetic field along the neutron propagation direction, and this definition does give reasonable results for motions through a free region and a square barrier. It is worth noting that in the zero magnetic field limit, the Larmor time coincides well with the CRT, which is due to the special shape of linear barrier, and may have some relevance to the weak equivalence principle. It is also found that the classical forbidden region plays an essential role for the dwell time $\tau_{_\mathrm{DW}}$ to match with the CRT, and the difference between the dwell and the phase times, \ie, the self-interference time delay, is barrier shape sensitive and clearly shows the peculiarity of the linear barrier. All the time delays are on the order of sub-millisecond and exhibit oscillating behaviors, signaling the self-interference of the scattering neutron, and the oscillations become evident only when the de Broglie wavelength $\lambda_k=2\pi/k$ is comparable to the characteristic length $L_c=[2m^2g/\hbar^2]^{-1/3}$. If the time delay measurement is experimentally realizable, it can probe the quantum nature for particle scattering off the gravitational potential in the temporal domain.