Quantum nonlinear optics with 2D materials

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When two beams from light torches cross, they do not clash like sabres from "Star Wars", but simply continue each its own way. This follows from the fact that free photons do not interact. However, when placed in an appropriate medium, photons can effectively feel the presence of each other, making the response of the optical system dependent on the number of photons. In this case, we say it has an optical nonlinearity provided by the medium. Typically the larger the volume, the stronger the nonlinearity, and the goal is to achieve prominent nonlinearity at the smallest possible scale. Together with the "sabre-effect", nonlinearity can ultimately provide the efficient manipulation of quantum states for photons. Thus with high level of nonlinearity single photon states can be prepared and used in quantum information processing. This would result in ultrafast quantum computing and communication platforms, serving as the basis for quantum applications that include secure communication networks, increased computational power and sensing at a level impossible to reach without quantum technologies.When light is confined in an optical cavity (for instance, set by two mirrors), its interaction with the medium is greatly enhanced. If the average number of roundtrips made by photons becomes large, they can hybridize with excitations in the medium, leading to half-light half-matter quasiparticles - polaritons. The hybridization makes confined light and the resulting polaritons able to interact. This ability stays behind the progress in numerous applications of classical nonlinear optics, including optical solitons for fast broadband communication. However, the task of finding an optimal system, where large nonlinearity for polaritons is achieved in the limit of few quanta, remains an open question.In the project, I will discover ways to increase optical nonlinearity at the minuscule scale. This will become possible by studying strong light-matter coupling in two-dimensional (2D) materials, where monolayer thickness can be smaller than a nanometer. Considering combinations of a few layers, I will show that the nonlinear response for polaritons can be elevated to the level where single photon processes become observable. The research will thus enable these easy-to-produce miniature systems for quantum optical processing to function as a platform for affordable quantum technologies.