Experimental Generation Of Squeezed Vacuum And Generation Scheme Of Hybrid Entanglement

Finite dimensional discrete variable and infinite dimensional continuous variable are two different ways of quantum information coding[1-2].In recent years,quantum information processing has made great progress in discrete variable[3-8]and continuous variable[9-14].However,both of them have their advantages and disadvantages[15-16].In the discrete variable system,the fidelity of the prepared and operated of the quantum state is close to 1,but the event is always probabilistic.In continuous variables,the preparation and operation of quantum states are deterministic,but quantum states are very sensitive to loss.In this context,the concept of hybrid entanglement is proposed.Hybrid entanglement combines the resources,operation and measurement of discrete variables and continuous variables to accomplish tasks that can not be accomplished by a single coding method[17-18].Hybrid entanglement is a hot topic in the field of quantum information and some leading researches have been reported.In 2013,Shuntaro Takeda et al.[19],from the university of Tokyo,Japan,demonstrated the deterministic teleportation of quantum bits of discrete variables experimentally by using the method of continuous/discrete variable hybrid operation.Then they realized the entanglement exchange between discrete variables and continuous variables.In 2016,AlexeyTiranov et al.[20]of the university of Geneva,Switzerland,experimentally observed the quantum correlation between microscopic single photons and macroscopic atomic assemblage.In 2014,OlivierMorin et al.[21],of gastronomic university in Paris,France,and HyunseokJeong et al.[22],of the national institute of optics,Italy,respectively realized hybrid entanglement of discrete variables and continuous variables by means of "photon subtraction" and "photon addition".The mixed entanglement of discrete variables and continuous variables is the basis of constructing hybrid quantum interface and has potential application value[23-24]in quantum communication and quantum computation.In this paper,some research work on hybrid entanglement is carried out.The optical "schrodinger" cat state,namely the coherent superposition state,can be used as the quantum bits of continuous variables and is an indispensable resource in the preparation of mixed entanglement.The optical"schrodinger" cat state can be prepared by squeezing the vacuum state and subtracting photons[25-28].We first carried out an experimental study on the generation of 795nm compressed vacuum state,795nm corresponds to the D1 transition line of rubidium atoms.,the preparation of this band of non-classical state provides a basis for future quantum relay.Then we analyze the generated continuous/discrete mixed entanglement[29-30],and find that the discrete quantum bits used are superposition states of vacuum state and single photon state,and this monorail quantum coding is sensitive to loss and not conducive to practical application.We propose a hybrid entanglement scheme for polarized qubits and coherent superposition states.In this scheme,polarized qubits are dual-track codes which can effectively resist loss in transmission and low detection efficiency.The main contents of this paper are:1.Vacuum compressed optical field of 795nm(rubidium atom D1 wire)was generated by optical parametric oscillation chamber(OPO).We used OPO cavity to measure the light field in the vacuum compressed state under the condition of different output cavity mirrors,at present,the highest compression degree measured in our experimental system is-3db and 4dB.On the basis of the existing vacuum squeezed states,we theoretically simulate the Wigner function of the phase space of the optical "schrodinger"cat state.The characteristic of the Wigner function is that it is obviously negative at the origin of the phase space,our work will provide a basis for the preparation of the atom-band optical "schrodinger" cat state.2.We propose a hybrid entanglement generation scheme between polarized qubits and coherent states.In this scheme,the interference between the coherent superposition state and the pair of polarized entangled photons is used to erase the path information and generate mixed entanglement.Since the polarization quantum ratio has two vertical polarization components,we designed two sets of interference devices with vertical polarization to predict the generation of mixed entanglement through the combined output of two sets of interference devices.We theoretically calculate the fidelity and probability of success of polarization mixing entanglement under non-ideal conditions,these non-ideal conditions include the use of an inefficient"on-off" single-photon detector and the availability of an approximate source as an input state.Theoretical analysis shows that,under actual experimental conditions,the fidelity of preparation of mixed entanglement(?f?1)can be kept at a high level.