Cosmological flux noise and measured noise power spectra in SQUIDs

Abstract

The understanding of the origin of $1/f$ magnetic flux noise commonly observed in superconducting devices such as SQUIDS and qubits is still a major unsolved puzzle. Here we discuss the possibility that a significant part of the observed low-frequency flux noise measured in these devices is ultimately seeded by cosmological fluctuations. We consider a theory where a primordial flux noise field left over in unchanged form from an early inflationary or quantum gravity epoch of the universe intrinsically influences the phase difference in SQUIDs and qubits. The perturbation seeds generated by this field can explain in a quantitatively correct way the form and amplitude of measured low-frequency flux noise spectra in SQUID devices if one takes as a source of fluctuations the primordial power spectrum of curvature fluctuations as measured by the Planck collaboration. Our model predicts flux noise with a spectrum given by an $1/f^{2-n_s}$ spectrum, where $n_s=0.96$ is the spectral index of the near-scale invariant primordial density fluctuations. For the typical amplitude of this cosmologically generated universal flux noise we theoretically calculate the average value $\delta \Phi /\Phi_0 =3.41 \cdot 10^{-6}$ at 1Hz. These theoretical predictions are in excellent agreement with recent low-frequency flux noise measurements of various experimental groups. Magnetic flux noise, so far mainly considered as a nuisance for electronic devices, may thus contain valuable information about fluctuation spectra in the very early universe.