Pure dephasing in flux qubits due to flux noise with spectral density scaling as $1/ f^α$
S. M. Anton, C. Müller, J. S. Birenbaum, S. R. O'Kelley, A. D. Fefferman, D. S. Golubev, G. C. Hilton, H. -M. Cho, K. D. Irwin, F. C. Wellstood, Gerd Schön, A. … 展开作者
S. M. Anton, C. Müller, J. S. Birenbaum, S. R. O'Kelley, A. D. Fefferman, D. S. Golubev, G. C. Hilton, H. -M. Cho, K. D. Irwin, F. C. Wellstood, Gerd Schön, A. Shnirman, John Clarke
For many types of superconducting qubits, magnetic flux noise is a source of pure dephasing. Measurements on a representative dc superconducting quantum interference device (SQUID) over a range of temperatures show that $S_Φ(f) = A^2/(f/1 \hbox{Hz})^α$, where $S_Φ$ is the flux noise spectral density, $A$ is of the order of 1 $μΦ_0 \, \hbox{Hz}^{-1/2}$ and $0.61 \leq α\leq 0.95$; $Φ_{0}$ is the flux quantum. For a qubit with an energy level splitting linearly coupled to the applied flux, calculations of the dependence of the pure dephasing time $τ_ϕ$ of Ramsey and echo pulse sequences on $α$ for fixed $A$ show that $τ_ϕ$ decreases rapidly as $α$ is reduced. We find that $τ_ϕ$ is relatively insensitive to the noise bandwidth, $f_1 \leq f \leq f_2$, for all $α$ provided the ultraviolet cutoff frequency $f_2 > 1/τ_ϕ$. We calculate the ratio $τ_{ϕ,E} / τ_{ϕ,R}$ of the echo ($E$) and Ramsey ($R$) sequences, and the dependence of the decay function on $α$ and $f_2$. We investigate the case in which $S_Φ(f_0)$ is fixed at the "pivot frequency" $f_0 \neq 1$ Hz while $α$ is varied, and find that the choice of $f_0$ can greatly influence the sensitivity of $τ_{ϕ,E}$ and $τ_{ϕ,R}$ to the value of $α$. Finally, we present calculated values of $τ_ϕ$ in a qubit corresponding to the values of $A$ and $α$ measured in our SQUID.
Qubit-photon interactions in a cavity: Measurement induced dephasing and number splitting
我们从理论上研究了电路QED结构中超导量子比特的测量诱导退相，并将结果与Schuster { \it et al.}的实验结果进行了比较，[Phys. Rev. Lett. 94，123602（2005）]。量子位与谐振器的强耦合导致量子位跃迁频率的显著ac-Stark偏移。结果，填充谐振器的光子数的量子波动导致量子比特的退相。我们发现量子比特谱的预测线形和实验结果之间有很好的一致性。此外，在强色散极限下，每个光子的斯塔克位移与腔衰减速率和量子比特线宽相比很大，我们预测量子比特光谱将分裂成多个峰，每个峰对应于腔中不同数量的光子。
Non-Gaussian dephasing in flux qubits due to 1/f-noise
最近的实验F。Yoshihara等人[Phys. Rev. Lett. 97，167001（2006）]和K. Kakuyanagi等人（cond-mat/0609564）提供了在各种偏置条件下约瑟夫森结通量量子比特中回波信号的退相干信息。这些结果被解释为假设高斯模型的退相干由于1/f噪声。在这里，我们重新审视这个问题的基础上，精确可解的自旋涨落模型再现的1/f噪声与量子比特相互作用的详细属性。我们考虑了回波信号的时间依赖性，并得出结论，基于高斯假设的结果需要重新考虑。
