  Secure communication protocols are becoming increasingly important, e.g. for
internet-based communication. Quantum key distribution allows two parties,
commonly called Alice and Bob, to generate a secret sequence of 0s and 1s
called a key that is only known to themselves. Classically, Alice and Bob could
never be certain that their communication was not compromised by a malicious
eavesdropper. Quantum mechanics however makes secure communication possible.
The fundamental principle of quantum mechanics that taking a measurement
perturbs the system (unless the measurement is compatible with the quantum
state) also applies to an eavesdropper. Using appropriate protocols to create
the key, Alice and Bob can detect the presence of an eavesdropper by errors in
their measurements. As part of the QuVis Quantum Mechanics Visualization
Project, we have developed a suite of four interactive simulations that
demonstrate the basic principles of three different quantum key distribution
protocols. The simulations use either polarized photons or spin 1/2 particles
as physical realizations. The simulations and accompanying activities are
freely available for use online or download, and run on a wide range of devices
including tablets and PCs. Evaluation with students over three years was used
to refine the simulations and activities. Preliminary studies show that the
refined simulations and activities help students learn the basic principles of
QKD at both the introductory and advanced undergraduate levels.
