3-Dimensional photonic circuits for quantum information processing

For many years silica fibers have been used for long-distance transfer of quantum information, and in particular within the domain of quantum communications and quantum key distribution. This is in part due to the ability to transmit tens of kilometers with negligible decoherence and loss. Although optical fibers fabricated with transparent materials such as SiO₂ allow for the necessary low-loss transmission, they lack the photon-photon nonlinearity that are necessary for implementation of the other aspect of quantum information, that of quantum computation. The necessary nonlinearity, that is weak in SiO₂, is stronger in material such as silicon nitride. In fact, silicon nitride has proven to be a promising platform for the generation and manipulation of single photons. The strong optical nonlinearity allows for parametric four-wave mixing which can be utilized to generate twin photons, while the transparency of silicon nitride in the near IR allows for low loss wave guides to be fabricated. However, due to silicon nitride\u27s dielectric nature and wide bandgap, it cannot be used as a photon counter - a necessary component for any quantum information protocol. In fact, in the visible and near-infrared, the detection of single-photons is possible using silicon avalanche photodiodes (SPADs). Silicon has proven to be a high-efficiency, high-speed, and low-cost single photon detector for wavelengths below 1 æm. Combining these two materials in a 3-D integrated circuit provides a platform capable of generating, routing, and detecting single photons. This 3-D device provides the transparency and nonlinearity of silicon nitride near IR, with the single-photon detection capability of silicon. These properties are ideal for future development of integrated quantum circuits for scalable quantum information processing applications. For the stoichiometric silicon nitride to act as a guiding layer it must be deposited using low pressure chemical vapor deposition (LPCVD). LPCVD results in a denser, high quality, sturdy silicon nitride film. If the silicon nitride is deposited using plasma enhanced chemical vapor deposition (PECVD) method the layer could have cracks throughout the film along with other impurities such as high hydrogen or excess silicon which can cause energy loss to the guided waves. Due to the dense and sturdy nature of LPCVD silicon nitride it can also act as a hard mask to protect the silicon photodetectors throughout the necessary thermal oxidation process. A wet thermal oxidation planarization technique is utilized in the fabrication of this photodector device. By thermally oxidizing the silicon the expensive chemical mechanical planarization technique can be avoided altogether. This also allows us to avoid chemical mechanical planarization (CMP) limitations such as wafer bow, curvature, and warp. We note here that our technique, due to the stress of the growth of the thermal oxide on the silicon nitride hard mask, shows some unavoidable imperfections, such as a bump known as a bird\u27s beak, which forms along the edges of the masked silicon