Study of directional couplers for optical qubit quantum operations

In this work, nanophotonic circuits with directional couplers are studied, which, along with phase shifters, are one of the main elements for creating circuits of a linear optical quantum computer. Directional couplers are two closely spaced waveguides, were made of silicon nitride. The subject of the study was the splitting ratio of directional couplers at a wavelength of 914 nm at room (300 K) and helium (3 K) temperatures. On the basis of experimental data, the coupling length at which all the power from one waveguide passes into another was extracted, and the refractive indices of silicon nitride at room (300 K) and helium temperatures (3 K) were found. The results of the work are of great importance in the design and manufacture of fully integrated quantum optical microcircuits with superconducting detectors operating at helium temperatures.