1201.5665 (Fei Yan et al.)

Spectroscopy of transverse and longitudinal low-frequency noise and
their temperature dependencies in a superconducting qubit    [PDF]

Fei Yan, Jonas Bylander, Simon Gustavsson, Fumiki Yoshihara, Khalil Harrabi, David G. Cory, Terry P. Orlando, Yasunobu Nakamura, Jaw-Shen Tsai, William D. Oliver

We report a direct measurement of the transverse and longitudinal
low-frequency noise that causes dephasing of superconducting flux qubits. The
former is due to flux fluctuations, and it affects the qubit's energy detuning;
the latter is parameterized as effective critical-current or charge
fluctuations, and affects the tunnel coupling between the qubit states. Our
method uses repeated single-shot measurements of the qubit state to resolve the
noise-power spectral density at frequencies limited only by the achievable
repetition rate. This approach enables an explicit measurement of the noise
over a range of frequencies that directly contributes to the dephasing observed
in ensemble-averaged time-domain measurements. For both noises, we determine
that the very same 1/f-type power laws measured at considerably higher
frequencies (0.2-20 MHz) are consistent with the 0.01-100-Hz range. We find no
evidence of temperature dependence of the noises over 65-200 mK, and also no
evidence of time-domain correlations between the two noises. These results help
us understand the properties of noise and dephasing pertinent to all
superconducting qubits.

View original: http://arxiv.org/abs/1201.5665