Physical layer security (PLS) is an attractive alternative to algorithmic security protocols, in that it is based on the physics of the communications channel rather than the "hardness" of particular mathematical problems. It is therefore inherently resistant to quantum computers. As an example of PLS, quantum key distribution (QKD) offers information-theoretic security, provided a number of assumptions are satisfied. However, QKD tends to generate secure keys at a fairly low rate, and it is limited in range. We consider instead classical encoding on quantized light, which is transmitted ova free-space optical channel, a protocol we have named "Semiclassical Photonic Key Distribution" or "SPhiKD." It can lead to significantly higher rates of key generation, in comparison with QKD, provided that a few physically-motivated constraints are placed on the eavesdropper. Research students will perform numerical analysis of this protocol with a goal of answering the following questions (and others): Can elements of QKD be added to SPhiKD to enhance the security of the protocol without sacrificing (much) bandwidth? Can our channel model be used for inventing improvements to QKD itself? In a realistic scenario (say, outdoors, during the daytime), will it be possible to engineer an optical system that can carry out SPhiKD with high fidelity? How can we optimize the protocol by judicious choice of physical encoding methods?