超导体产生的倏逝波 Johnson 噪声
Evanescent-wave Johnson Noise from Superconductors
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中文总结 AI 辅助
该研究计算了超导体上方的倏逝波 Johnson 噪声及其对邻近自旋和电荷量子比特弛豫时间的影响，揭示了相干峰增强和磁性杂质导致的零温噪声基底。
中文摘要 AI 辅助
我们计算了超导体上方真空半空间中的倏逝波 Johnson 噪声（EWJN），以及由此产生的、放置在距表面纳米距离处的自旋和电荷量子比特的弛豫时间（$T_1$）。电磁响应由 BCS 超导体的单一微观横向电流响应核 $Q(q, \omega)$ 描述。该响应核在任意频率和温度下，以及波矢 $q \ll k_F$（费米波矢）条件下，针对不同密度的非磁性杂质和磁性杂质进行了计算。当与涨落-耗散定理和半空间的非局域表面阻抗相结合时，这给出了距表面任意距离 $z \gg k_F^{-1}$ 处的磁场和电场噪声，由此我们得到 $T_1$。在略低于 $T_c$ 时，由于耗散电导率的相干（Hebel-Slichter 型）峰，磁噪声相对于正常态增强，并在较低温度下呈指数下降；电噪声没有相干峰。该理论预测存在由磁性杂质引起的零温噪声基底。在由对破缺产生的无隙区中，有限的亚隙态密度 $\nu(0)$ 产生与温度无关的噪声谱密度，并且弛豫率受 $T_1^{-1}(T)\le[\nu(0)/\nu_F]^{2}\\,T_{1,N}^{-1}(T)$ 限制（对于 $T\ll T_c$），在极端非局域区域取等号。这里 $\nu(0)$ 和 $\nu_F$ 分别是费米能量处超导态和正常态的态密度，$T_{1,N}(T)$ 是相同电极在相同温度下处于正常态时产生的弛豫时间。
英文摘要
We compute the evanescent-wave Johnson noise (EWJN) in the vacuum half-space above a superconductor, and the resulting relaxation time ($T_1$) of spin and charge qubits placed at nanometer distances from the surface. The electromagnetic response is described by a single microscopic transverse current-response kernel $Q(q, ω)$ for a BCS superconductor. This is computed for varying densities of both non-magnetic impurities and magnetic impurities, for arbitrary frequency and temperature and for wave vectors $q \ll k_F$ (the Fermi wavevector). When combined with the fluctuation-dissipation theorem and the nonlocal surface impedances of the half-space, this yields the magnetic and electric field noise at any distance $z \gg k_F^{-1}$ from the surface, from which we obtain $T_1$. Just below $T_c$ the magnetic noise is enhanced relative to the normal state by the coherence (Hebel-Slichter-type) peak of the dissipative conductivity and drops exponentially at lower temperatures; the electric noise shows no coherence peak. The theory predicts that there is a zero-temperature noise floor induced by magnetic impurities. In the gapless regime produced by pair breaking, the finite subgap density of states $ν(0)$ yields a temperature-independent noise spectral density and a relaxation rate bounded by $T_1^{-1}(T)\le[ν(0)/ν_F]^{2}\,T_{1,N}^{-1}(T)$ for $T\ll T_c$, with equality in the extreme nonlocal regime. Here $ν(0)$ and $ν_F$ are the superconducting and normal-state densities of states at the Fermi energy, and $T_{1,N}(T)$ is the relaxation time the same electrode would produce in its normal state at the same temperature.
发表机构
- University of Wisconsin-Madison（威斯康星大学麦迪逊分校）
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