Comparison between simulated and measured spectra of spontaneous parametric down-conversion photons in AlGaAs Bragg...

This work investigated the spectral properties and state fidelity of photons generated by spontaneous parametric down-conversion in AlGaAs Bragg-reflection waveguides. Effective refractive indices, group indices, their dispersion, and their influence on the spectral properties of the down-converted photons were evaluated by simulation and characterization of fabricated waveguides. A comparison was made between two different types of waveguides differing in the ridge width, specifically narrow ridges of ∼1.7-1.9 μm and wide ridges of ∼4 μm. The group indices for the down-converted and pump photons were determined by Fabry-Pérot measurements. Group indices dispersion and the ratio between the group indices of signal and idler photons as well as the expected state fidelity were extracted from measured spectra of the down-converted photons. Additionally, the fidelity to the Bell state was determined by quantum state tomography. For larger Δn_g we observe substantial deviation between theoretical spectra and fidelity beyond the experimental uncertainties.