  Learning quantum mechanics is challenging, even for upper-level undergraduate
and graduate students. Research-validated interactive tutorials that build on
students' prior knowledge can be useful tools to enhance student learning. We
have been investigating student difficulties with quantum mechanics pertaining
to the double-slit experiment in various situations that appear to be
counterintuitive and contradict classical notions of particles and waves. For
example, if we send single electrons through the slits, they may behave as a
"wave" in part of the experiment and as a "particle" in another part of the
same experiment. Here we discuss the development and evaluation of a
research-validated Quantum Interactive Learning Tutorial (QuILT) which makes
use of an interactive simulation to improve student understanding of the
double-slit experiment and strives to help students develop a good grasp of
foundational issues in quantum mechanics. We discuss common student
difficulties identified during the development and evaluation of the QuILT and
analyze the data from the pretest and post test administered to the upper-level
undergraduate and first-year physics graduate students before and after they
worked on the QuILT to assess its effectiveness. These data suggest that on
average, the QuILT was effective in helping students develop a more robust
understanding of foundational concepts in quantum mechanics that defy classical
intuition using the context of the double-slit experiment. Moreover, the
upper-level undergraduates outperformed physics graduate students on the post
test. One possible reason for this difference in performance may be the level
of student engagement with the QuILT due to the grade incentive. In the
undergraduate course, the post test was graded for correctness while in the
graduate course, it was only graded for completeness.
