Fabrication of an integrated optical resonator for microwave to optical conversion on an atom chip

Currently quantum computing has proven to make fast and accurate logical calculations and promises to drastically change the field of computing. One of the main limitations are the short coherence times of qubits, which precludes information storage, and the possibility of making distant communications between quantum registers that is challenging using microwave photons (these are resonant with qubit transitions). The MOCA project proposes the use of an integrated chip combining superconducting resonators with optical waveguides and cavities that converts the microwave photons to optical photons. The chip is then coupled to an ensemble of cold atoms for the long term storage of the information. In this way, the fabrication of a Radio Frequency (RF) resonator and a photonic resonator play an important role in the creation of the device. The project is part of the QuantERA programme, created to develop quantum technologies in Europe. However, the tasks to accomplish the goal are divided into 5 research groups in which experimental and theoretical physics are applied. INRIM is in charge of fabricating the RF and photonic resonators, and in my thesis I focused on developing the photonic components. The resonator consists of 3 parts: a waveguide in which the optical photons are confined, a cavity with bragg reflectors to create a resonator for efficient photon conversion and a grating to couple the signal from the waveguide to the optical fibers. The operating wavelength chosen is 760 nm. For the good confinement of the wave in the waveguide, a high refractive index material with low losses is needed. We chose Silicon Nitride (SiN), and we modified the recipe for deposition in order to increase the refractive index up to 2.4. The SiN thin films are deposited by Chemical Vapor Deposition (CVD) on a dielectric substrate (in this case, thick corning glass). We calibrated the deposition process and measured the deposition rate in order to obtain a final thickness of 200 nm, which is the thickness required for the waveguides. We optimized the final geometry for the waveguides and the gratings by means of a Finite Element Method commercial software and the obtained structures were replicated in a CAD software for Electron Beam Lithography (EBL). The lithographic process was followed by an Aluminum deposition to obtain an hard mask that could be used in a Reactive Ion Etching step to remove the exceeding Silicon Nitride. For the RIE step we optimized a recipe that approximates to a Silicon Etching recipe more than a SiN recipe, and taking into account the need of a conformal structure, a pseudo-bosch etching was used. Secondly is the grating coupler, whose parameters can be calculated considering the angle of incidence of the light into the grating and the Bragg’s condition for the proper diffraction of the light. Finite Element Method (FEM) modeling was performed to optimize the structure. Thirdly, we want to confine light in a microcavity by fabricating Distributed Bragg Reflectors (DBR) along the waveguide. In such a way, we want to increase the photon density within the cavity and enable the conversion of microwave photons radiated by the cold atom ensemble into optical ones. As an alternative route for the light confinement and manipulation, we also considered using a metasurface made of SiN nanopillars. FEM models show that such structures can sustain resonant modes with a quality factor as high as 10^5.