High performance photon-pair source based on a fiber-coupled periodically poled KTiOPO₄ waveguide

Abstract

Photon-pair sources based on spontaneous parametric downconversion (SPDC) in a nonlinear crystal waveguide have been shown to be significantly more efficient than those in a bulk crystal. To utilize waveguide sources in quantum information processing (QIP) applications, it is highly desirable to integrate additional functionality such as pump sources and modulators at the waveguide-chip level for compactness, reliability, and ease of operation. As a first step we develop a waveguide SPDC source with integrated single-mode polarization-maintaining (PM) fibers in this thesis work, and demonstrate the efficient generation of photon pairs at 1316 nm in a type-II phasematched Rb-indiffused waveguide in periodically poled KTiOPO4 (PPKTP). We perform the flux and spectrum characterization of our integrated waveguide source, and obtain a pair production rate of 2 x 107/s/mW in a 1.08-nm bandwidth. The measurement results are in good agreement with a theoretical model that takes into account the transversal momentum imparted on the phase matching function by the waveguide. With narrowband filtering and a pump power, we achieve a Hong-Ou-Mandel quantum-interference visibility of 98.2% after subtraction of accidental coincidences, representing the highest reported value for a waveguide-based photon-pair source. The photon-pairs generated by our PPKTP waveguide are shown to be highly indistinguishable, in terms their spectra and spatial modes. Therefore the fiber-coupled waveguide source is particularly suitable for long-distance quantum communication protocols such as fiber-based quantum key distribution (QKD).