Investigation of the origin of the low-frequency noise in liquid nitrogen rf SQUIDs

Sebastian Kempf, Anna Ferring, A. Fleischmann +1 more

Dongfeng He, Hideo Itozaki

We simulate the dielectric SrTiO3 (STO) resonator for radio frequency superconducting quantum interference devices (rf SQUIDs) using electromagnetic analysis software. The simulated values of the resonant frequencies, the unloaded quality factor, the loaded quality factor under 50Ω matching, and the coupling with the SQUID are in good agreement with the experimental results. The simulation also shows that the strength of the electromagnetic field outside the resonator decreases exponentially with the distance from the resonator, which proves that the field is confined to a small volume near the resonator. Due to the strong temperature dependence of the permittivity of STO, the resonant frequency changes with the temperature fluctuations of liquid nitrogen, which may cause the low frequency noise of rf SQUIDs with dielectric resonators.

Michael Mück, Christoph Heiden, John L. Clarke

A detailed study has been made of the low-frequency excess noise of rf superconducting quantum interference devices (SQUIDs), fabricated from thin niobium films and operated at 4.2 K, with rf bias frequencies of 0.15, 1.7, and 3 GHz. When the SQUIDs were operated in an open-loop configuration in the absence of low-frequency flux modulation, the demodulated rf voltage exhibited a substantial level 1/f noise, which was essentially independent of the rf bias frequency. As the rf bias frequency was increased, the crossover frequency at which the 1/f noise power was equal to the white noise power moved to higher frequencies, because of the reduction in white noise. On the other hand, when the SQUID was flux modulated at 50 kHz and operated in a flux locked loop, no 1/f noise was observed at frequencies above 0.5 Hz. A detailed description of how the combination of rf bias and flux modulation removes 1/f noise due to critical current fluctuations is given. Thus, the results demonstrate that the 1/f noise observed in these SQUIDs is generated by critical current fluctuations, rather than by the hopping of flux vortices in the niobium films.

瑞史 松田

T. Becker, Michael Mück, Christoph Heiden

We have prepared n-channel silicon field-effect transistors, which are capable of working at liquid helium temperatures (4.2 K) and used them in cooled preamplifiers for rf superconducting quantum interference device (SQUID) readout electronics. All metallizations of these transistors were made of niobium, to study the possibility of a further integration of a SQUID and FET on the same chip. Using the FETs in a cooled preamplifier together with a rf SQUID gradiometer, the flux noise of the system could be reduced by a factor of 3 compared to a room temperature low noise preamplifier. We have also performed calculations of a possible increase of the substrate temperature due to the power dissipation of the FET and have measured the cross talk between FET and SQUID.

V. I. Shnyrkov, Georgy M. Tsoi