Off-resonant Raman quantum memory in impurity crystals: signal-to-noise ratio analysis

The possibility of implementing an off-resonant Raman scheme of optical quantum memory on the basis of an ensemble of three-level atoms is investigated under the condition of equal polarisations of resonant transitions forming the Λ-scheme. The developed model is used to analyse the signal-to-noise ratio at the output of an optical quantum memory device in ¹⁴³Nd³⁺ : Y ⁷LiF₄. It is shown that this ratio can significantly exceed unity for single-photon input pulses. The required values of the parameters can be obtained by using an impurity crystal in the form of a whisperinggallery mode ring resonator.