Narrowband quantum sources of polarization-correlated photon pairs at 800-nm Wavelength.

Free space is crucial for secure, long-range quantum communication, enabling high-fidelity entanglement-based quantum network interconnections with enhanced security. Spontaneous Parametric Down Conversion (SPDC) is an efficient process for generating entangled photons utilized as qubits through three-wave mixing. Second Harmonic Generation (SHG) is the classical reverse process of SPDC. As a characterization tool for an efficient SPDC source, we have experimentally examined three (10mm,20mm&30mm) periodically poled PPKTP crystals in a type-II process (H+V→H) @800nm wavelength. We have Verified the effect of focusing using three different lenses, demonstrating that efficiency increases quadratically (from 0.032% to 0.24% per Watt) with crystal length while the phase matching temperature (PMT) decreases while increasing the focusing parameter. We generated correlated photon pairs via SPDC using SHG data using three PPKTP crystals in a type-II process (H→H+V) @400nm wavelength. By measuring the coincidence counts at different signal/idler detuning controlled via a tunable filter, we obtained the coincidence spectrum leading to their joint spectral intensity (JSI) and spectral brightness. We made a comparative analysis of our results pertaining to the three PPKTP crystals of different lengths based on these experimental results.