Generation of Entanglement in Optical Fibers Using SPDC in a Sagnac Interferometer

Sammanfattning

This thesis presents the characterization of SagnaQ, a fiber-based polarization-entangled photon pair source developed at Quantum Scopes AB. The source generates photon pairs at telecom wavelengths through type-0 Spontaneous Parametric Down-Conversion (SPDC) in a Periodically Poled Lithium Niobate (PPLN) waveguide crystal placed inside a Sagnac interferometer loop. The purely fiber-coupled design moves towards a compact, plug-and-play platform, offering greater stability and ease of use compared to free-space implementations.The generated quantum state was characterized through Quantum State Tomography (QST), polarization correlation measurements, a Clauser-Horne-Shimony-Holt (CHSH) inequality test, and heralded second-order correlation function measurements. The reconstructed density matrix yields a fidelity of $85\,\%$ to the $|\Phi^{-}\rangle$ Bell state, with a purity of $0.759 \pm 0.008$ and a concurrence of $0.715 \pm 0.008$. Polarization correlation visibilities of $0.929 \pm 0.018$ and $0.790 \pm 0.011$ were measured in the HV and DA bases respectively, and the CHSH parameter was found to be $S = 2.3301 \pm 0.0075$, violating the classical bound by more than 44 standard deviations. A heralded second-order correlation function of $g_{H}^{(2)}(0) = 0.087 \pm 0.002$ confirms the single-photon character of the source. Power-dependent measurements demonstrate a linear scaling of coincidence count rates with pump power, with a degradation of state quality observed at higher pump powers.