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Improving Students' Understanding of Quantum Mechanics via the Stern-Gerlach Experiment

The Stern-Gerlach experiment has played a central role in the discovery of spin angular momentum. It can also play a pivotal role in teaching the formalism of quantum mechanics using a concrete example involving a finite-dimensional Hilbert space. Using this context, students can learn about how to prepare a specific quantum state starting from an arbitrary state, issues related to the time evolution of the wave function, and quantum measurement. It can also be exploited to teach students about the distinction between the physical space where one performs the experiment and the Hilbert space where the state of the system lies and how the information about the state of the system in the Hilbert space can be exploited to interpret the possible outcomes of the experiment in the physical space. Students can learn the advantages of choosing an appropriate basis to make suitable predictions about the outcomes of experiments with different arrangements of Stern-Gerlach devices. This experiment can also help students understand that an ensemble of identically prepared systems, e.g., one in a linear superposition of two stationary states, is not the same as a mixture, e.g., in which half of the systems are in one stationary state and the other half are in the other stationary state. Here, we discuss investigation of students' difficulties about the Stern-Gerlach experiment by giving written tests and interviewing advanced undergraduate and graduate students in quantum mechanics courses. We also discuss preliminary data from two quantum mechanics courses that suggest that a Quantum Interactive Learning Tutorial (QuILT) related to the Stern-Gerlach experiment is helpful in improving students' understanding of these concepts.