Generation And Detection Of Entangled Photon Pairs For Quantum Computing

Quantum computing is one of the most important new computing techniques in post era of Morre.The characteristic size of transistor on integrated circuit is getting smaller and smaller,following Moore's law.When the characteristic size becomes comparable with atoms,the corresponding quantum effects would make classical circuits no more reliable.The concept of quantum computing put forward by Feynman in 1981 provides computing techniques with a new direction of development.Based on quantum superposition and quantum entanglement,quantum computing follows rules of quantum mechanics;thus,it can solve the problems of quantum effects.It has been proved theoretically,that quantum computing can achieve an exponential speed-up for some specific problems.Entangled photons are the most critical sources for the implementation of optical quantum computing,which meet with the problems of “decoherence” and “low brightness”.The KLM scheme put forward by Knill,Laflamme and Milburn in 2001 has proved that,it is possible to realize optical quantum computing with entangled photons with post selection.Most of the present implementation schemes of optical quantum computing are based on it,with entangled photon as the most important sources.Unfortunately,the problem of decoherence will cause the loss of entanglement,and that of low brightness will make photons no more fit for quantum bit.This article mainly studied the technique of generation and detection of entangled photons based on type-I SPDC.The main work and contributions are listed as follows:1.Aiming to solve the problem of decoherence,we modeled the variation of phases of entangled photon pairs,and designed Mathematica computing programs.We have found that the spatial and temporal phase differences are the main causes of decoherence.We have also calculated the function of compensation crystals,which can eliminate decoherence,providing theoretical guidance for the design of entangled photon pairs with strong coherence.2.Aiming to solve the problem of low brightness,we modeled the propagation of photons in free space and inside nonlinear crystals.We have found that the focused beam waist radius and cutting angle of nonlinear crystals are the main factors which affect brightness,and got proper radius and cutting angle for experiment,providing theoretical guidance for the design of entangled photon pairs with high brightness.3.We have put forward a scheme for the detection of photons,with time window finely adjustable.We have also designed schematic diagram of the corresponding circuit,providing a technic approach for detecting entangled photon pairs with high efficiency.4.Based on type-I SPDC,we have designed and realized two photons entangled in polarization.The experimental result shows that,the value for CHSH inequality is 2.76 with a violation by 13 standard deviation,and visibility of 97.89%,verifying the quantum entanglement between these two photons.