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arXiv:1509.04084 · Physics Education Research Conference (PERC)

Student difficulties with quantum states while translating state vectors in Dirac notation to wave functions in position and momentum representations

Emily Marshman, Chandralekha Singh

Published Dec 18, 2015• 15 citations
arXivHTMLPDF

Abstract

Dirac notation is often used in upper level quantum mechanics courses, but students struggle with this representation.To investigate the difficulties that advanced students (i.e., upper-level undergraduate and graduate students) have while translating state vectors in Dirac notation to wave functions in position and momentum representations, we administered free-response and multiple-choice questions and conducted individual interviews with students.We find that students display common difficulties with these topics.

DOI
10.1119/perc.2015.pr.048

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Dirac notation is often used in upper level quantum mechanics courses, but students struggle with this representation.To investigate the difficulties that advanced students (i.e., upper-level undergraduate and graduate students) have while translating state vectors in Dirac notation to wave functions in position and momentum representations, we administered free-response and multiple-choice questions and conducted individual interviews with students.We find that students display common difficulties with these topics.
Dirac notation is used commonly in quantum mechanics.However, many upper-level undergraduate and graduate students in physics have difficulties with representations of quantum operators corresponding to observables especially when using Dirac notation.To investigate these difficulties, we administered free-response and multiple-choice questions and conducted individual interviews with students in advanced quantum mechanics courses.We discuss the analysis of data on the common difficulties found. I.
Abstract Learning quantum mechanics (QM) is widely recognized as a challenging task, as it involves highly abstract concepts and requires a significant shift from classical physics reasoning. Developing expertise in QM requires students to represent physical concepts using multiple formalisms, such as Dirac notation and algebraic representations, and to move flexibly between them. This study investigates two main categories of student difficulties related to Dirac notation. The first concerns difficulties in expressing a generic state vector in the position and momentum representations. The second relates to challenges in writing the eigenvectors of the position and momentum operators in Dirac notation. To explore these issues, we administered questionnaires to third-year undergraduate physics students enrolled in a QM course at the Lyon 1 University. We present a detailed analysis of student responses, identify the principal difficulties encountered, and compare our findings with results reported in prior American studies. Our results show that many students struggle to establish a clear connection between Dirac notation and its functional algebraic representation. In particular, students find it easier to recognize state vectors and eigenvectors than to construct them explicitly. Additionally, common difficulties include confusion between bras (e.g. x | and p |) and the corresponding operators, as well as confusion between the Dirac delta distribution and the Kronecker delta. Finally, we propose some interpretations of our results in light of those reported in previous studies.
We discuss an investigation of upper-level undergraduate and graduate students' difficulties with fundamental concepts involving a system of identical particles.The investigation was carried out in advanced quantum mechanics courses by administering written questions and conducting individual interviews with students.We find that students share many common difficulties related to these quantum mechanical concepts, e.g., with regard to the many-particle Hamiltonian and stationary state wavefunctions for a system of non-interacting identical particles.
We investigate the difficulties of advanced undergraduate students toward the end of a full year upper-level quantum mechanics course with concepts related to quantum measurements and time development. Our analysis is based upon a test administered to 89 students from six universities and interviews with 9 students. Strikingly, most students shared the same difficulties despite variations in background, teaching styles, and textbooks. Concepts related to stationary states, eigenstates, and time dependence of expectation values were found to be particularly difficult. An analysis of written tests and interviews suggests that widespread misconceptions originate from an inability to discriminate between related concepts and a tendency to overgeneralize.
Quantum mechanics is challenging even for advanced undergraduate and graduate students.In the Schrödinger representation, the wave function evolves in time according to the time dependent Schrödinger equation.The time dependence of the wave function gives rise to time dependence of the expectation value of observables.We have been exploring the difficulties that advanced undergraduate and graduate students have with time dependence of expectation values in quantum mechanics.We have developed and administered conceptual free response and multiplechoice questions to students to investigate these difficulties.We also interviewed 23 students individually using a thinkaloud protocol to obtain a better understanding of the rationale behind students' written responses.We find that many students struggle with time dependence of expectation values of observables.We discuss some findings.