Quantum Network Based On Atomic Ensemble

Quantum information science is an interdisciplinary subject of quantum mechanics and information science.In recent years,with the rapid development of quantum information technology,the quantum network has attracted more and more attentions.Quantum network usually consists of quantum nodes and quantum channels.Its implementation requires the generation and characterization of quantum coherence and entanglement among quantum nodes.The basis of quantum network is quantum interconnection,which converts quantum states from one physical system to another in a reversible manner.The quantum connectivity can be achieved by the interaction between light and matters,thus allowing the distribution of entanglement and transmission of quantum states among quantum nodes.Therefore,the construction of quantum network requires the preparation of quantum states that propagate among quantum nodes,i.e.the preparation of non-classical light fields that can interact with quantum nodes.Then the nonclassical light interacts with quantum nodes to realize the transfer of quantum states.It is useful to construct the non-local entanglement.In continuous-variable regime,the generation of non-classical light field is usually realized through optical parametric processing in an optical cavity,and the non-classical properties of light field are described by the quadrature amplitude and quadrature phase or polarized components of light field.Both the polarized component of light field and the spin wave of atoms can be described by Stokes operators,and the polarized components of light is more conducive to interact with atoms.The measurement of polarized components of light field does not need local light,so we carry on the preparation and application of polarized non-classical light field.Quantum state is the basic element of quantum information processing.In long-distance quantum communication,the non-classical properties of quantum states will decrease due to the loss introducing by the externalenvironment.Quantum repeater,which combines the quantum storage,quantum purification and entanglement swapping,can overcome the losses.Atomic nodes can be used as quantum repeaters.Quantum storage can transfer quantum states among different physics system,it provides favorable condition for quantum network combining the entanglement purification and entanglement swapping.The completed main research works are as following:1.Establishing the experimental preparation system of non-classical optical fields,including polarized squeezed and polarized entangled states.The system consists of an external frequency doubling cavity and three optical parametric amplifiers.The second harmonic generated by the external-cavity frequency doubling system is used as the pump light of the optical parametric amplifiers.In the experiment,we use the degenerated optical parametric amplifiers prepare the orthogonal squeezed,the EPR entangled state and the GHZ entanglement.And transfer them into the polarized squeezing and entanglement states by the polarized beam splitting prism.2.The scheme is proposed theoretically to realize entanglement between atomic ensembles.Firstly,Raman interaction is used to realize the mixed entanglement between the light and atoms,and together with entanglement swapping conquering the transmission loss to establish the entangled between atomic ensembles.3.In the experiment,we generate the tripartite GHZ entangled state light field,and store the tripartite GHZ entangled state into the three rubidium atomic ensembles with the 2.6 meters distance by the method of EIT mechanism,and establish the quantum entanglement among the three atomic ensembles.Then,the stored entangled state is released through three classical optical channels.Finally,the entanglement among atomic ensembles is verified by measuring the correlation variance of the released light fields.4.The development of quantum network depends on the high-quality entanglement among quantum nodes,and inevitable decoherence limits the entangled quality.We propose a scheme for continuous-variable entanglement distillation,the EPR entanglement is distributed through quantum channels to two independent atomic ensembles to establish the atom-atom entanglement,and then another EPR entanglement interfere with the two atomic ensembles which are entangled to perform entanglement distillation.The scheme requires only atomic ensemble quantum storage and balanced homodyne detection,and overcomes the phase noise caused by long distance transmission.And it can be extended to multi-partite atomic ensembles.5.The high efficient quantum storage can be used in quantum computing,quantum network and entanglement purification.A theoretical model of cavity-enhanced continuous-variable quantum storage is presented,and in experiment we put the thermal atom ensemble in an optical resonator to improve the optical storage efficiency by enhancing the interaction between light and atoms.The creative works are as follows:1.The polarized non-classical light field corresponding to the D1 absorption line of rubidium atom can interact directly with the atomic ensembles,and the measurement of quantum noise of polarized components does not need local oscillating light.The experimental scheme can be extend to the generation of multi-partite polarized entanglement state.2.By the method of transferring quantum state from light to atoms,we have firstly realized the deterministic entanglement among three long distance atomic ensembles in the experiment.Moreover,the additional noise introduced by the experimental scheme is small,and the scheme can be extended to multiple quantum nodes.3.Continuous-variable entanglement distillation scheme for atomic ensembles is put forward in theoretical,which only requires Gaussianoperation and balanced homodyne detection technology.This method avoids complex non-Gaussian operation and single photon detection.