Why the Afshar Experiment Does Not Refute Complementarity

TL;DR

This paper argues that the Afshar experiment does not violate quantum complementarity, clarifies misconceptions about 'which-way' measurements, and shows its consistency with established quantum interpretations and delayed choice experiments.

Contribution

It provides a detailed analysis demonstrating that the Afshar experiment aligns with quantum principles and clarifies the misleading terminology used in its interpretation.

Findings

The Afshar experiment does not refute complementarity.

The experiment is analogous to spin pre- and post-selection scenarios.

Misleading terminology has contributed to confusion in interpreting the experiment.

Abstract

A modified version of Young's experiment by Shahriar Afshar demonstrates that, prior to what appears to be a ``which-way'' measurement, an interference pattern exists. Afshar has claimed that this result constitutes a violation of the Principle of Complementarity. This paper discusses the implications of this experiment and considers how Cramer's Transactional Interpretation easily accomodates the result. It is also shown that the Afshar experiment is analogous in key respects to a spin one-half particle prepared as ``spin up along $\bf x$'', subjected to a nondestructive confirmation of that preparation, and post-selected in a specific state of spin along $\bf z$. The terminology ``which-way'' or ``which-slit'' is critiqued; it is argued that this usage by both Afshar and his critics is misleading and has contributed to confusion surrounding the interpretation of the experiment. Nevertheless, it is concluded that Bohr would have had no more problem accounting for the Afshar result than he would in accounting for the aforementioned pre- and post-selection spin experiment, in which the particle's preparation state is confirmed by a nondestructive measurement prior to post-selection. In addition, some new inferences about the interpretation of delayed choice experiments are drawn from the analysis.