Improvement Of Entanglement For Entangled Coherent States And Their Properties

As non-Gaussian operations such as photon addition and photon subtraction can be used to improve the non-classicalities including entanglement of quantum states,thus these operations and their applications are widely concerned.In this thesis,we used quantum operations and their superposition operations to enhance the degree of entanglement of the entangled coherent states,and the corresponding non-classicalities.These quantum states can be prepared by combining physical devices such as beam splitter and can be expected to promote the development of quantum state engineering.Firstly,two mode excited entangled coherent states(TME-ECSs)are introduced by operating repeatedly the photon excited operator on the ECSs.It is shown that the normalization constant is related to the product of two Laguerre polynomials.The influence of the operation on nonclassical behaviour of the ECSs is investigated in terms of cross correlation function,antibunching effect and the negativity of Wigner function,which show that nonclassical properties can be enhanced.In addition,inseparability properties of the TME-ECSs are discussed by using Bell inequality and concurrence.It is found that the degree of quantum entanglement of even ECSs increases with the increase of the total excited photon number,and the violation of Bell inequality can be present for both even and odd case only when the total excited photon numbers are even and odd,respectively.Secondly,a new entangled quantum state is introduced by applying local coherent superposition(CS)tara)(++of photon subtraction and addition to each mode of an even entangled coherent state(EECS).The properties of entanglement are investigated in terms of the Einstein Podolsky Rosen(EPR)correlation,the degree of entanglement,and the average fidelity of quantum teleportation.It is found that single-and two-mode CS operations can improve the EPR correlation of the EECS in a big(>88.0)and small(<0.52)region of amplitude,respectively.The enhanced entanglement and fidelity depend on the even or odd order of CS.All optimal results are not achieved at two extreme cases of t=1,0.Finally,a scheme is proposed to generate a new kind of non-Gaussian state--theLaguerre polynomial excited coherent state(LPECS)--by using multiphoton catalysis with coherent state input.The nonclassical properties of the LPECS are studied in terms of nonclassical depth,Mandel's parameter,second-order correlation,quadrature squeezing,and the negativity of the Wigner function(WF).It is found that the LPECS is highly nonclassical and its nonclassicality depends on the amplitude of the coherent state,the catalysis photon number,and the parameter of the unbalanced beam splitter(BS).In particular,the maximum degree of squeezing can be enhanced by increasing the catalysis photon number.In addition,the effect of decoherence is examined by using the WF,which shows that the negative region,the characteristic time of decoherence,and the structure of the WF are affected by catalysis photon number and the parameters of the unbalanced BS.Our work provides a general analysis on how to prepare such polynomial quantum states,which may be useful in the fields of quantum information and quantum computation.