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Applied Physics Letters

Parameter fluctuations and low frequency noise in Josephson junction devices

Claudia D. Tesche

Published Jul 1, 1982• 15 citations
Publisher

Abstract

A model is presented for a possible source of low frequency (1/f) noise in the dc superconducting quantum interferometer device (SQUID). Thermal fluctuations within the device are assumed to generate low frequency fluctuations in the device parameters. The resultant parameter fluctuations induce fluctuations in the device characteristics. As an example, low frequency voltage noise spectral densities are computed as a function of temperature and applied flux. In addition, low frequency variations in the forward transfer function of the SQUID are predicted. The model can be generalized to predict low frequency noise generated by parameter fluctuations in other Josephson junction circuits.

DOI
10.1063/1.93303

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A model is presented for a possible source of low frequency (1/f) noise in the dc superconducting quantum interferometer device (SQUID). Thermal fluctuations within the device are assumed to generate low frequency fluctuations in the device parameters. The resultant parameter fluctuations induce fluctuations in the device characteristics. As an example, low frequency voltage noise spectral densities are computed as a function of temperature and applied flux. In addition, low frequency variations in the forward transfer function of the SQUID are predicted. The model can be generalized to predict low frequency noise generated by parameter fluctuations in other Josephson junction circuits.
We have studied low frequency (1/f) noise of YBa2Cu3O7−δ dc superconducting quantum interference devices (SQUIDs) on SrTiO3 bicrystal substrates. 1/f flux noise, either measured at different temperatures for optimized bias current or measured at 77 K for different bias currents, is almost constant. These facts imply that 1/f noise mainly comes from fluctuations of the critical current of the Josephson junction that form the SQUID. Also, we explain the critical current fluctuations in the junction by an equilibrium temperature fluctuation model.
We have measured the low-frequency noise in the critical current Ic of six dc superconducting quantum interference devices (SQUIDs) with resistively shunted Nb–NbOx–PbIn Josephson junctions in the temperature range T=0.09–4.2K. Each device is voltage biased, the applied flux is an integer number of flux quanta, and the current fluctuations are measured with a second dc SQUID. At low frequencies f, there is a component of the power spectrum of the critical current fluctuations given approximately by SIc(f)=CIc2T2∕Af, where A is the area of both junctions, and C≈(3.9±0.4)×10−23m2∕K2. For quantum bits based on Josephson junctions, the scaling of SIc(f) with T2 implies that the dephasing time limited by critical current l∕f noise should scale as 1∕T for temperatures down to at least 0.09K.
The noise characteristics at low frequencies are studied for various planar dc SQUIDs made of junctions with different barrier materials of a-Si, aluminum oxide, and niobium oxide. It is found that fluctuations of the parameters of individual junctions contribute to the flux noise of the SQUIDs in a magnitude of the same order with the apparent flux noise. The magnitude of the flux noise due to the parameter fluctuations depends on the barrier material and on the inductance parameter βL . We have observed a telegraph noise in the voltage time trace and a Lorentzian in the voltage power spectrum for a current-biased SQUID. As a source of the parameter fluctuations, capture and emission of electrons at the trapping sites in the barrier are indicated.
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The authors investigated low-frequency noise in RF and DC superconducting quantum interference devices (SQUIDs) with step-edge Josephson junctions (SEJs). The noise properties have been measured as a function of frequency, temperature, bias current, and magnetic field. The low-frequency noise has a telegraph-like character in certain conditions and depends quasi-periodically on flux and temperature. This telegraph noise was also observed in single step-edge junctions. The experimental results were explained by a model of such a junction in which an array of parallel and series connected weak links acts like a two-level fluctuator. This fluctuator arises from the quantum states of a multijunction SQUID with spatially inhomogeneous SEJs resembling a multijunction array.>
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