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arXiv 0810.1334cond-mat.supr-con
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二能级系统的饱和与约瑟夫森结量子比特中的电荷噪声

Saturation of Two Level Systems and Charge Noise in Josephson Junction Qubits

Magdalena Constantin, Clare C. Yu, John M. Martinis

摘要:

研究了在衬底中电偶极矩涨落二能级系统(TLS)中产生的约瑟夫森量子比特中的电荷噪声$S_Q$。TLS由角频率为$Ω$且电场强度为$I$的交变电场驱动。没有广泛认识到,如果$I\gg I_c$,则小量子位中的TLS可以容易地强烈饱和,其中$I_c$是临界电场强度。为了研究饱和对电荷噪声的影响,我们用密度矩阵元表示噪声谱密度。为了确定密度矩阵元素对比率$I/I_c$的依赖性,我们使用Bloch-Redfield微分方程找到密度矩阵的稳态解。然后,我们得到一个表达式作为频率$f$和比率$I/I_c$的函数的电荷波动的谱密度。我们发现$1/f$电荷噪声在低频率下,并且电荷噪声在高频率下是白色的(恒定的)。使用平坦的态密度,我们发现TLS饱和在高或低频率的电荷噪声没有影响。

英文摘要:

We study the charge noise $S_Q$ in Josephson qubits produced by fluctuating two level systems (TLS) with electric dipole moments in the substrate. The TLS are driven by an alternating electric field of angular frequency $Ω$ and electric field intensity $I$. It is not widely appreciated that TLS in small qubits can easily be strongly saturated if $I\gg I_c$, where $I_c$ is the critical electric field intensity. To investigate the effect of saturation on the charge noise, we express the noise spectral density in terms of density matrix elements. To determine the dependence of the density matrix elements on the ratio $I/I_c$, we find the steady state solution for the density matrix using the Bloch-Redfield differential equations. We then obtain an expression for the spectral density of charge fluctuations as a function of frequency $f$ and the ratio $I/I_c$. We find $1/f$ charge noise at low frequencies, and that the charge noise is white (constant) at high frequencies. Using a flat density of states, we find that TLS saturation has no effect on the charge noise at either high or low frequencies.

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