Experimental Generation Of Nonclassical Light Field In Rubidium Atomic Band

The variables in quantum mechanics are divided into separated variables and continuous variables according to their intrinsic states.Their applications in quantum information have their own characteristics^[1-3].The quantum states of separated variables are insensitive to loss and has high fidelity,but the production is probabilistic^[4].The generation and operation of quantum states of continuous variables are deterministic,but has low fidelity.Besides it is extremely sensitive to loss.In recent years,the study of hybrid entanglement has become a hotpot in the field of quantum information.The hybrid entanglement able to combine the advantages of continuous variables and separated variables to accomplish tasks that can not be accomplished by one of the variables^[5].In 2013,the group of A,Furusawa demonstrated the deterministic quantum separation of discrete variable Qubit by means of continuous variables^[6].After that,the group realized the entanglement exchange between the separated variables and the continuous variables^[7].In2014,the group of Julien Laurat realized the mixed entanglement of single photon and cat states in the experiment^[8].Our group intends to carry out the research of hybrid entanglement.This paper study on the generation of non-classical light field in 795nm band,which includes the narrow band polarized entangled photon pairs and the squeezed vacuum state.This provides the basis for the research of the hybrid quantum entanglement and quantum repeater.The main contents of this paper are as follows:?1?Using class?periodically polarized titanium oxyphosphate crystal?PPKTP?as nonlinear medium,through the process of spontaneous parametric down conversion?SPDC?,the polarization-dependent photon pairs of 795 nm has been produced.The PPKTP crystal is put into the optical cavity,and the line width of the generated photon pairs are limited by the cavity.Single longitudinal mode output of down conversion photon pairs is accomplished by cascaded etalons.After single photon detection,the photon pairs is projected to the polarized entangled state.And the entangled state is not maximally entangled due to an additional phase shift between the different polarization directions of photon pairs.In order to generate the maximum entangled state,we compensate the additional phase by using the phase compensation system.The measured Bell parameter S=2.6±0.07which violates the Bell-CHSH inequality by⁸.6 standard deviations^[9].The down conversion photon pairs has a line width of 15 MHz,which can effectively combined with atomic.?2?The squeezed vacuum state of 795 nm is generated by using a parametric oscillation cavity?OPO?based on periodically polarized titanium oxyphosphate crystal.The squeezed vacuum state is detected by a balanced homodyne detection system and the squeezing degree of-3dB is obtained.The time-domain signal of the balanced homodyne measurement system is collected on the oscilloscope,and the noise distribution of the squeezed vacuum state at different phase angles is obtained.The maximum likelihood estimation method is used to reconstruct the squeezed vacuum state,and its Wigner function is obtained^[10].?3?The influence of the imperfection of experimental conditions on the generation of Schrodinger cat state is calculated theoretically^[11],which includes the imperfection of the squeezed state and the detector.The Wigner function of the cat state obtained by subtracting photons from the squeezed vacuum state is obtained.The expression of the function includes the squeezing degree and squeezing purity of the squeezed vacuum state,the dark counts and quantum efficiency of the single photon detector,and the total efficiency of the balanced zero-beat detection.According to the obtained theoretical model,the effects of different experimental conditions on the generation of cat state were analyzed in detail.It provides a theoretical basis for the next step to obtain Schrodinger cat state by subtracting photons from squeezed vacuum state.