Pure dephasing in flux qubits due to flux noise with spectral density scaling as $1/ f^α$

Details Pure dephasing in flux qubits due to flux noise with spectral density scaling as $1/ f^α$ Pure dephasing in flux qubits due to flux noise with spectral density scaling as $1/ f^α$

2011-11-30

arxiv.org/abs/1111.7272v2

Physics

Condensed Matter

Superconductivity

7 pages, 8 figures, 1 table

Scientific paper

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 $S_\Phi(f) = A^2/(f/1 \hbox{Hz})^\alpha$, where $S_\Phi$ is the flux noise spectral density, $A$ is of the order of 1 $\mu\Phi_0 \, \hbox{Hz}^{-1/2}$ and $0.61 \leq \alpha \leq 0.95$; $\Phi_{0}$ 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 $\tau_\phi$ of Ramsey and echo pulse sequences on $\alpha$ for fixed $A$ show that $\tau_\phi$ decreases rapidly as $\alpha$ is reduced. We find that $\tau_\phi$ is relatively insensitive to the noise bandwidth, $f_1 \leq f \leq f_2$, for all $\alpha$ provided the ultraviolet cutoff frequency $f_2 > 1/\tau_\phi$. We calculate the ratio $\tau_{\phi,E} / \tau_{\phi,R}$ of the echo ($E$) and Ramsey ($R$) sequences, and the dependence of the decay function on $\alpha$ and $f_2$. We investigate the case in which $S_\Phi(f_0)$ is fixed at the "pivot frequency" $f_0 \neq 1$ Hz while $\alpha$ is varied, and find that the choice of $f_0$ can greatly influence the sensitivity of $\tau_{\phi,E}$ and $\tau_{\phi,R}$ to the value of $\alpha$. Finally, we present calculated values of $\tau_\phi$ in a qubit corresponding to the values of $A$ and $\alpha$ measured in our SQUID.

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