Keywords: 
• 量子存储-压缩热态光场-保真度

Abstract: Quantum memory of light is not only the building block of constructing large-scale quantum computer,but also the kernel component of quantum repeater for quantum networks,which makes long distance quantum communication come true.Due to the inevitable optical losses,squeezed vacuum generated from optical parametric amplifier becomes squeezed thermal state of light,which is no longer the minimum uncertainty state.Therefore quantum memory of squeezed thermal state of optical field is the key step towards the implementation of quantum internet.Atomic ensemble is one of ideal quantum memory media,as a result of high optical depth and good atomic coherence.Electromagnetically induced transparency (EIT) is one of mature approaches to quantum state mapping between non-classical optical fields and atomic spin waves.In atomic ensembles,the EIT can on-demand map quantum state between quadratures of light and spin waves of atomic ensemble,i.e.,controlled quantum memory.Here the condition of quantum memory for squeezed thermal state of light is investigated according to the fidelity benchmark of quantum memory.The fidelity benchmark of quantum memory is the maximum fidelity which can be reached by classical methods,and it is quantum memory if the memory fidelity is higher than the fidelity benchmark of quantum memory.By numerically calculating the fidelity benchmark of quantum memory for different kinds of squeezed thermal states of light and the dependence of memory fidelity on the memory efficiency,we obtain the minimum memory efficiency which can realize quantum memory for squeezed thermal state of light.The quantum memory can be easily obtained by increasing squeezing parameter r.The thermal state fluctuation is sensitive to the realization of quantum memory.The required minimum memory efficiency is lower,when smaller thermal state fluctuation is employed in experiment by reducing the optical losses in optical parametric amplifier.On the other hand,quantum memory fidelity benchmark is high for small squeezing parameter and large optical depth,which requires high memory efficiency.And atomic memory efficiency can be increased by utilizing optical cavity to enhance the interaction between light and atom or atomic ensemble with high optical depth.For example,the fidelity benchmark is 0.80,when squeezing parameter r is 0.35 and thermal state fluctuation is 2.38 dB.Thus quantum memory can be realized if the memory efficiency is larger than 4.34％.Our work can provide the direct reference for experimental design of continuous variable quantum memory,quantum repeater,and quantum computer based on atomic ensembles.