Physics – Condensed Matter – Superconductivity
Scientific paper
2008-10-08
Physics
Condensed Matter
Superconductivity
13 pages, 9 figures, accepted for publication in Phys. Rev. B
Scientific paper
10.1103/PhysRevB.79.094520
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 $\Omega$ 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.
Constantin Magdalena
Martinis John M.
Yu Clare C.
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