Non-Gaussian Low-Frequency Noise as a Source of Qubit Decoherence

We study decoherence in a qubit with the distance between the two levels affected by random flips of bistable fluctuators. For the case of a single fluctuator we evaluate explicitly an exact expression for the phase-memory decay in the echo experiment with a resonant ac excitation. The echo signal as a function of time shows a sequence of plateaus. The position and the height of the plateaus can be used to extract the fluctuator switching rate $\gamma$ and its coupling strength $v$. At small times the logarithm of the echo signal is $\propto t^3$. The plateaus disappear when the decoherence is induced by many fluctuators. In this case the echo signal depends on the distribution of the fluctuators parameters. According to our analysis, the results significantly deviate from those obtained in the Gaussian model as soon as $v\gtrsim \gamma$.

Figure 1

Schematic distribution of charged traps located near the gate surface and producing oppositely charged images.

Figure 2

Echo signal for different values of the ratio $v/2\gamma$ (shown near the curves), Eq. (6). Dashed lines—calculations along the Gaussian assumption, Eq. (7).