超导离子芯片近场热噪声引起的离子加热研究

Ion heating induced by near-field thermal noise of superconducting ion chip

基于涨落耗散定理研究了不同温度下超导离子芯片中近场热噪声引起的离子加热效应,通过电场格林函数和多层介质反射系数得到了不同情况下电场涨落谱密度的近似式。对于净铌电极,当温度T为295 K时,近场热噪声谱密度与电极厚度成反比;T为4 K时,近场热噪声谱密度与电极厚度无关,且通过计算发现其近场热噪声相较于室温下降了十几个数量级。分析了铌电极表面存在Nb_2O_5薄膜的情况,结果表明Nb_2O_5层薄膜引起的电场涨落占据主要地位,噪声谱密度与Nb_2O_5厚度成正比,芯片温度降至4 K时其热噪声下降5～6个量级。

Ion heating effect incuced by near-field thermal noise of superconducting ion chip at different temperature is investigated based on fluctuation-dissipation theorem. The approximate expressions of electric field fluctuation spectral density under different conditions are obtained by the electric field Green function and multi-layer reflection coefficients. For pure Nb electrode, the near-field thermal noise spectral density is inversely proportional to the electrode thickness when the temperature T is 295 K. The near-field thermal noise spectral density is irrelevant with the electrode thickness when T is 4 K, and the near-field thermal noise can be reduced by over ten orders of magnitude comparing to the case of room temperature. The case that there are Nb2O5 thin films on the surface of niobium electrode is analyzed. Results show that the electric field fluctuation induced by Nb2O5 film occupies the main position. The noise spectral density is proportional to the Nb2O5 thickness. When chip temperature decreases to 4 K, the thermal noise decreases by 5 to 6 orders of magnitude.