Clear and rigorous quantum reasoning is needed to explain quantum physical phenomena. As pillars of true quantum physical explanations, we suggest specific quantum reasoning derived from quantum physical key ideas. An experiment is suggested to support such a quantum reasoning, in which a quantized radiation field interacts with an optical beam splitter, leading to experimental results conflicting with classical physical predictions. The results, however, can be explained consistently with a quantum reasoning based on the key ideas of probability, superposition, and interference (PSI). In this quantum optical key experiment the optical beam splitter prepares a superposition of single photon states and a Michelson interferometer is used to detect the superposition via controlled propagation phases. Although different single photon experimental setups (aimed at helping students to gain access to foundational issues in quantum physics) have been discussed in the past, the wave-particle dualism bound to classical physics maintains its predominance as an explanation pattern for the interpretation of these experiments. The study presented here investigates the effect of the quantum optical key experiment on the ability of students to use quantum reasoning based on the key ideas of PSI to overcome the naive wave-particle dualism. The current state of relevant studies that test student access to quantum physics can roughly be divided into two distinct areas: one tests how mathematical abilities help them to understand quantum physics and one tests how nonmathematical representations of a set of specific quantum theoretical traits (“Wesenszüge”) lead to a deeper understanding of quantum physics. There is a lack of questionnaires that focus on the idea of developing quantum reasoning based on superposition, probability, and interference of quantum states combined with a real experiment using true quantum light. In the first part of the article, we describe the physical modeling and present the development of the questionnaire. The set of items has been constructed from newly developed items and combined with well-tested ones. The validation of the set addresses qualitative and quantitative methods. In the second part, we give a pre- and poststudy examination of the impact of the quantum optical key experiment on students’ quantum reasoning. A significant increase in the number of students using quantum arguments is based on PSI reasoning for the explanation of an interference, such as the behavior of single photon states. Though the increase is significant, we found only minor changes in a particular issue to the students’ reasoning when approaching quantum physics as illustrated by a sample of answers given in the second part of the article. The concept of quantum states and the principle of superposition still appear particularly difficult.

• Received 2 January 2023
• Revised 13 December 2023
• Accepted 28 February 2024

DOI:https://doi.org/10.1103/PhysRevPhysEducRes.20.010122

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.

Published by the American Physical Society