Overcoming Atmospheric Effects in Quantum Cryptography

Quantum Computers will have the potential to greatly assist us in problems such as searching, optimization and even drug discovery. Unfortunately, among these newfound capabilities is one which allows one to break RSA encryption in orders of magnitude less time. One promising countermeasure to secure our communication today and in the future is the one time pad, although it is very difficult to generate and distribute. Quantum Key Distribution offers a practical method for two authenticated parties to generate a key. Whereby the parties, Alice and Bob, share quantum states and use physical laws to place an upper bound on the information an eavesdropper could possibly have. QKD has matured tremendously since it was proposed in 1984, but gaps still remain between the device models used in security proofs and what can be achieved in deployments. In particular, the atmosphere presents a challenging environment for QKD, and due to the wireless nature of communications today, we cannot avoid it. QKD must be robust to atmospheric loss as well as transmittance fluctuation due to turbulence. In this dissertation, I perform an experiment to make the BB84 protocol more resilient by monitoring the transmittance of a channel and finding time periods where the transmittance is low, hence error rate is high and discard data collected during such timeframes. We conduct an experiment to test Hong Ou Mandel visibility, which is at the heart of Measurement Device Independent QKD, by varying the detector parameters and the photons' level of identity. I use an observation where we find that the interference effect is highly dependent on the similarity of photon numbers at the measurer's beamsplitter to develop a scheme in MDI QKD where the key rate is kept high through tethering the transmittance fluctuations of Alice and Bob's channels with dynamic attenuation. The results show an improved robustness when in a turbulent atmosphere, even when accounting for nonzero minimum loss.