Research On Quantum Entangled Sources Based On Optical Parametric Process

In the past two decades,quantum information science developed rapidly and has become one of the most popular research fields.Quantum information science is a large class of disciplines,which contains many research directions,for example,quantum computing,quantum cryptography,quantum communication,etc.Quantum entangled sources are an indispensable component in all quantum information science research directions.Entanglement is an important feature in quantum mechanics,which describes the non-classical correlation between particle pairs.Even if the entangled pairs of particles are separated by a great distance,the quantum states of each particle in the system cannot be described separately from the state of the other particles.All particles in the entanglement system must be treated as a whole.At present,there are two main design schemes for optical quantum entangled sources:one is spontaneous parametric down-conversion based on second-order nonlinear susceptibility,and the other is four-wave mixing based on third-order nonlinear susceptibility.Since the third-order nonlinear susceptibility is much smaller than the second-order nonlinear susceptibility,the four-wave mixing process is not conducive to the preparation of high-bright entangled photon pairs.Therefore,we design two-photon entanglement sources using the spontaneous parametric down-conversion based on second-order nonlinear susceptibility.In this thesis,we have developed a design procedure for a secondorder nonlinear photonic crystal using the double lattice method.By entering the number and phase mismatch vectors of required parametric processes,the procedure can design a nonlinear photonic crystal that satisfies the required parametric processes.In theory,our nonlinear photonic crystal design program can satisfy any form of optical spontaneous parametric down-conversion process.In the design process of photon entangled sources,we firstly design the spontaneous parametric down-conversion processes and calculate their phase mismatch vectors.Next,we design the nonlinear crystal which quasi-phase matches the required parametric processes.We designed a second-order quasi-periodic nonlinear photonic crystal using our nonlinear photonic crystal design program.This nonlinear photonic crystal can simultaneously quasi-phase match two spontaneous parametric down-conversion processes.In one of the spontaneous parametric down-conversion processes,the signal photons are transmitted in the positive direction,and the idle photons are transmitted in the reverse direction.The other parametric process is the opposite of the previous one.The signal photons are transmitted in the opposite direction,and the idle photons are transmitted in the forward direction.These two spontaneous parametric down-conversion processes are collinear in the crystal and their coefficients of occurrence are the same.Through the quasi-periodic nonlinear photonic crystal,we can achieve the maximum counterpropagating path-entangled two-photon state.In the two-photon path entangled source we designed,the photon pair has a stable output direction,which not only ensures the stability of the entangled state,but also facilitates the collection of entangled photon pairs.We simulated the spectrogram of the maximum counterpropagating path-entangled two-photon state.Compared to copropagating entangled sources,the counterpropagating path-entangled sources result in a narrower and purer spectrum.Our path entangled source is not only suitable for quantum coding and quantum computing,but also has great potential in the research of integrated photonic chips.