Controlling structured light: Nonlinear caustics and high-dimensional QKD
Dr. Robert Fickler (Max Planck Centre for Extreme and Quantum Photonics, University of Ottawa, Canada)

I my talk I will present two recent nonlinear and quantum optics experiments involving structured light. In the first experiment, we investigate nonlinear effects on the formation on caustic pattern. Caustics can be seen as nature’s way to focus energy without using a lens. They are also connected to rogue or freak waves, which have extremely large amplitudes, appear more often than a normal distribution would predict and have been considered to be fairy tales of sailors until they have been scientifically recorded in 1995. We investigate the effect of nonlinearity on these phenomena by generating optical caustics and rogue wave events in linear and nonlinear media. We find that in linear media only large phase fluctuations generate strong caustics and rogue wave events. However, if light experiences nonlinearity, caused by the response of passing through a Rubidium vapour, even small fluctuations lead to strong caustics and enlarged probabilities for rogue wave events. Because oceans are also described by a third order nonlinearity, our results may not only hold for optical systems but also be helpful in understanding of oceanic rogue waves and the forecast of the effects of tsunami waves. In the second experiment, we structure the transverse shape of single photons to generate high-dimensional quantum states, which enable an enlarged information content and are known to have a better noise resistance in quantum cryptography applications. We encode 4-dimensional states using a combination of spin and orbital angular momentum degree of freedom of light and investigate the influence of turbulence after the transmission over a 300m long intra-city link in Ottawa. We find that although moderate turbulence effects the achievable key rates in a slightly negative manner, it is still possible to establish a secure key with a higher rate than an analogous 2-dimensional qubit-state allows via the same link. While this is a promising first step to high-dimensional long-distance quantum communication, adaptive optics to compensate for wave distortions will become indispensable to cover longer distances and transmit higher rates in the future.