Perturbative study of wave function evolution from source to detection of a single particle and the measurement

TL;DR

This paper investigates the evolution of a single particle's wave function from source to detector, analyzing the effects of apertures and perturbations to understand wave function collapse and phase information.

Contribution

It introduces a perturbative method using a thin pin to probe wave function collapse and phase information during particle detection.

Findings

Wave function envelope has a spindle shape with pointed ends at the slits.

Aperture slit distorts phase information, which can be preserved using a thin pin.

The perturbative function is real-valued and closely related to the wave function.

Abstract

We analyze the evolution of a particle wave function when it propagates through free space in the longitudinal z-direction from a thin entrance slit to a detector behind a thin exit slit parallel to the horizontal y-axis. We consider an extra aperture slit between the two slits to probe the evolution of the wave function and close the aperture slit starting from wide open until the detection counting rate in a repeated experiment drops to half. When all the slits are long and thin, the 1D Schroedinger equation gives the wave function evolution until the final detection. The width of the aperture slit in the vertical x-direction depends on the z-position of the slit providing an approximate description of the wave function evolution. The width of the function characterizing this dependence starts from the entrance slit. It grows wider until it reaches a maximum and then shrinks narrower and finally collapses into the exit slit where the particle is detected. Thus the envelope of this function has a spindle shape with its pointed ends at the two slits. Hence it is very different from the well-known wave function of the Schroedinger equation with the initial condition at the entrance slit, which is narrow only at the beginning, then grows wider until it reaches the exit slit, where it is much larger than the slit width. However, the phase information is lost because the aperture slit distorts the wave function. To keep the phase information, we replace the aperture slit with a thin pin (parallel to the y-axis) that blocks the wave function. We then study its perturbative effect on the counting rate of the detector. This analysis provides a function to probe the process of the wave function collapse right before the detection. We show this function is real-valued, with amplitude and phase information, and is closely related to the wave function.