The Study Of Quantum Photonic Chip For Polarization Entanglement Based On Lithium Niobate Waveguide Circuits

The polarization of photon is a unique carrier for quantum qubit due to the fact that it is easy for operation and robust to the decoherence.The polarization entanglement has been one of the most common and widely used entangled states in the field of quantum information.The polarization entangled photons can be prepared from different material systems based on the second or third order nonlinear optical effect.At present,one of the commonly used methods is the spontaneous parametric down conversion process in the second order nonlinear crystals.This method is simple and easy,but most of the related studies are based the bulk crystals or periodically poled waveguide.This thesis is devoted to the engineering of polarization-entangled photons on a single chip.This on-chip scheme will greatly improve the photon flux,the spatial mode,the stability,the miniaturation,the integration and the operability when compared with the bulk crystals,hence will greatly enhance the quality of polarization entanglement.The on-chip quantum light source is an inevitable tendency toward the practical applications of quantum information.It is the primary task for realizing the complex quantum information processing and further the fully integrated quantum chip.The main results of this thesis include the following:1.We have successfully fabricated the single mode Ti-diffused lithium niobate waveguide with a low propagation loss.The mechanism of Ti-diffusion at high-temperature was studied and the reason of waveguide loss was investigated.By the comprehensive and detailed study of physical conditions for the lithography,the thickness of the titanium strip and the difusion time,we successfully prepared the single mode Ti-diffused lithium niobate waveguide by optimizing the variety of parameters.We also characterize the waveguide loss2.We have successfully poled the Ti-diffused lithium niobate waveguide.By improving the room-temperature poling technique,we successfully realized the ferroelectric domain inversion in Ti-diffused lithium niobate waveguide.On this basis,we designed the poling period to fulfill the quasi-phase matching condition.We successfully observed the second-order harmonic generation and the spontaneous parametric down conversion.The poling period in the waveguide was checked and established3.We propose and design the on-chip polarization beam splitter based on lithium niobate waveguide.According to the theory of plannar waveguide and the electro-optic effect of lithium niobate,the three-waveguide coupled mode theory is derived following the similar procedure in the two-waveguide coupling.The distribution of power in three waveguides is deduced.Due to the difference between Ti-diffused and proton-exchanged lithium niobate waveguides,the theoretical scheme of polarization beam splitter on lithium niobate was proposed.Also the working principle of another type of polarization beam splitter which is called zero-gap waveguide is analyzed and the numerical simulation is carried out.4.We proposed and designed the Half-Wave plate based on periodic poling lithium niobate waveguide.By designing the poling period region and applying appropriate voltage,we can achieve the on-chip half-wave plate and further realize the manipulation of qubit.In the experiment,we have made the first attempt of the on-chip half-wave plate and made some test on the polarization transition ratio.5.We have designed the overall structure of polarization-entangled source,and analyzed the working principle and the overall performance.We conceived a method to obtain the internal loss,the splitting ratio etc.by studying the single photon couting rate and the coincidence counting rate.We take the on-chip path-entangled source as the example.By solving a set of equations,we can get the performance of each component on the chip.This supplies as a nondestructive and convenient way.