  The Stern Gerlach experiment has played a central role in the discovery of
spin angular momentum and it has also played a pivotal role in elucidating
foundational issues in quantum mechanics. Here, we discuss investigation of
students' difficulties related to the Stern Gerlach experiment by giving
written tests and interviewing advanced undergraduate and graduate students in
quantum mechanics. We also discuss preliminary data that suggest that the
Quantum Interactive Learning Tutorial (QuILT) related to the Stern Gerlach
experiment is helpful in improving students' understanding of these concepts.
