Generation Of Frequency-entangled Photon Pairs Based On A Dual-periodically-poled Ti:LiNbO₃ Waveguide

Quantum mechanics is the most important cornerstone of modern physics,which can be used to deal with phenomena that cannot be explained by classical physics.Quantum information science is the combination of quantum physics and information science with qubit as the basic information unit,and mainly comprises quantum communication and quantum computation.In recent years,remarkable achievements have been made in quantum information science.Quantum entanglement is the core resource of quantum information,and has broad applications in a variety of quantum information tasks,including quantum key distribution,quantum teleportation,quantum computation,and quantum metrology.We can predict that in the near future,large-scale integrated quantum information processing based on quantum entanglement can have broader applications.Integrated photonic quantum information technology has developed rapidly in recent years,aiming to integrate the quantum state engineering,manipulation,measurement and other optical elements on a single chip with a footprint of several square centimeters or even smaller,which can benefit for the stability and scalability of quantum information processing?QIP?.However,most of the currently reported photonic chips are passive,and an off-chip quantum light source is needed.Hence,how to prepare an efficient integrated quantum entangled photon source has become our research focus.Optical superlattices are a kind of material widely used to generate entangled-photons at present.By designing the domain structure of optical superlattices,the quasi-phase-matching condition can be satisfied for the spontaneous parameter down-conversion process,which can produce entangled photon pairs in an efficient way.The periodically-poled lithium niobate?PPLN?crystal is an ideal nonlinear material with high nonlinear coefficients and mature manufacturing and integration techniques.PPLN waveguides can meet the requirements of integrated entangled photon source well as the pump and parametric light can be confined well,and thus has a higher conversion efficiency.Since the entangled photon pairs generated from spontaneous parameter down-conversion meet the momentum and energy conservation conditions,they can be entangled in many degrees of freedom such as polarization,frequency,and path.The frequency degree of freedom can form a scalable high-dimensional Hilbert space and can improve quantum information coding capacity.Hence investigations on frequency-entangled photon pairs are of importance and great application prospect.The main content of this dissertation includes the following aspects:1.In the introduction,we briefly introduce the research background of quantum information and calculate the spontaneous parametric down-conversion?SPDC?process of the nonlinear crystal as well as the phase-matching conditions.We also review the related research progress.2.Photon interference is caused by the indistinguishability of photons.We briefly describe the basic principles of single-photon and multi-photon interference based on linear optical elements.3.LiNbO₃ is the most widely used nonlinear crystal.We briefly introduce the fabrication processes of LiNbO₃ waveguide,and theoretically calculate the Hong-Ou-Mandel?HOM?beating pattern of the frequency-entangled photons generated by the SPDC process.4.We design and experimentally realized a dual-periodically-poled Ti:LiNbO₃?Ti:PPLN?waveguide for generating frequency-entangled photon pairs,where two nondegenerate type-II quasi-phase-matching spontaneous parametric down-conversion processes can be satisfied simultaneously.The dual-periodically-poled Ti:LiNbO₃waveguide is able to create entangled-photons directly without interference or post selection with a single pump laser,as it can provide two different inverse lattice vectors for the SPDC process.The frequency-entangled photon pairs are characterized by the HOM beating interference with a high fidelity and a high entanglement degree demonstrated.Our approach opens up an efficient way for integrated entangled source,which may be applied to active quantum photonic chips.