  We describe the development of a Quantum Interactive Learning Tutorial
(QuILT) on quantum key distribution, a context which involves a practical
application of quantum mechanics. The QuILT helps upper-level undergraduate
students learn quantum mechanics using a simple two state system and was
developed based upon the findings of cognitive research and physics education
research. One protocol used in the QuILT involves generating a random shared
key over a public channel for encrypting and decrypting information using
single photons with non-orthogonal polarization states, and another protocol
makes use of two entangled spin-1/2 particles. The QuILT uses a guided approach
and focuses on helping students build links between the formalism and
conceptual aspects of quantum physics without compromising the technical
content. We also discuss findings from a preliminary in-class evaluation.
