Quantum high-dimensional communication with orbital angular momentum structured light

Structured light and metaoptics are two emerging fields that have recently received increasing interest: the former allows to exploit more degrees of freedom of photons, such as the orbital angular momentum (OAM); the latter has revolutionized optics by transforming bulk elements into thin, planar and multifunctional components, which can be integrated into the semiconductor manufacturing. Combining OAM light and silicon metasurfaces, new applications stand out for both classical and quantum communications to meet the growing demand for transmission capacity and security. High-dimensionality of the structured light allows, indeed, the upgrade of quantum protocols such as Quantum Key Distribution, to increase the information rate, the robustness against errors and eavesdropping of third parties. The purpose of this study is the simulation, design, fabrication and optical characterization at classical and single-photon regime of a silicon metasurface for the processing of light beams carrying both spin and orbital angular momentum in the telecom infrared. The proposed sorter implements the log-pol optical transformation for OAM demultiplexing and, at the same time, it is also polarization-sensitive, due to the metasurface design which exploits the geometric Pancharatnam-Berry phase, implemented with continuously variant subwavelength gratings. The sorter is projected to be compact and flat and its nanofabrication consists of two main steps: the creation of a high-resolution master with electron beam lithography and the manufacture of copies through fast and low-cost imprinting and inductively coupled plasma-reactive ion etching, thus acquiring the technological possibility of mass-production. The entire process is finely tuned to optimize diffraction and efficiency. Finally, both the classical and the single-photon characterizations show promising results that leave open prospects for an increase of the transmission capacity via spatial (de)multiplexing and for the realization of a prototype bench for quantum high-dimensional secure communication.