In this project, we utilize two different approaches to prepare polarization entangled photon sources. In the first method, we design the duty cycle of the periodically poled non-linear crystal. The crystal thus generates non-degenerate polarization entangled photons by simultaneously achieving two distinct types of phase-matching conditions. The wavelengths of the emitted photons locate at 1074 nm and 1081 nm with the pump laser at 538.6 nm. Also, the poling period is designed to be 2 mm, which effectively alleviates the influence of the fabrication error of duty cycle of the crystal. By simulation with Monte Carlo method, we find that both the average fidelity and concurrence stay higher than 0.98 even if the fabrication error is 100μm.

To verify the quality of the entanglement, we firstly exploit CHSH inequality (Bell inequality) to inspect the non-locality of the photon state. The result with 84 standard deviations beyond the classical limit represents the explicit violation to the inequality, implying that the correlation between the photon pair doesn’t satisfy local-hidden-variable (LHV) theorem. Furthermore, we reconstruct the density matrix of the state by applying quantum state tomography (QST). The reconstructed quantum state shows a fidelity F=0.998 and a concurrence C=0.934 comparing to the Bell state. These results provide confident evidence of high quality polarization entanglement of the photons.

The second approach of generating polarization entangled photons is to employ the Sagnac interferometer. By placing a non-linear crystal at the center of the interferometer, the oppositely propagating pump laser will probabilistically generate photon pair in each direction. After the two wavefunctions converge on the polarization beam splitter (PBS), the output state will perform a polarization entangled state. The wavelengths of the photons exploiting in this approach are 1535 nm and 1560 nm, with the pump laser is centered at 773.8 nm. The quality of entanglement of emitted photon pair is also certified by CHSH inequality and QST. In this approach, we have also observed the violation of CHSH inequality, the fidelity F=0.992 and concurrence C=0.891. After the preparation, we forward one of the photons via a 300 m long outdoor single mode optical fiber to another building. The distributed photon pair also present F=0.972, C=0.856 and the violation of CHSH inequality. The adequate entanglement performance of the photon pair after the distribution under non-laboratory environment exhibit the feasibility of the quantum network technologies.