Magnetism in SQUIDs at Millikelvin Temperatures

Abstract

We have characterized the temperature dependence of the flux threading dc SQUIDs cooled to millikelvin temperatures. The flux increases as 1/T as temperature is lowered; moreover, the flux change is proportional to the density of trapped vortices. The data are compatible with the thermal polarization of surface spins in the trapped fields of the vortices. In the absence of trapped flux, we observe evidence of spin-glass freezing at low temperature. These results suggest an explanation for the universal 1/f flux noise in SQUIDs and superconducting qubits.

Evidence for temperature-dependent spin diffusion as a mechanism of intrinsic flux noise in SQUIDs

“…1 In the case of qubits, this magnetic flux noise places fundamental limits on the performance and scalability of such architectures. Low frequency flux noise is widely thought to be due to fluctuations of magnetic impurities local to the superconductor wiring [2][3][4] but the identity of these impurities and the physical mechanism producing the observed fluctuations is not known. Understanding the fundamental origin of flux noise is important not only to aid in its reduction in superconducting devices, but also may provide insight into the behavior of disordered ensembles of spins at low temperature.…”

“…Understanding the fundamental origin of flux noise is important not only to aid in its reduction in superconducting devices, but also may provide insight into the behavior of disordered ensembles of spins at low temperature. Indeed, flux noise experiments 4 suggested the presence of a spin-glass phase, motivating further computational studies of fluctuations in spin-glasses. 7 Some likely candidates for spin impurities include dangling-bonds in the oxide surrounding the superconducting wire, 8 or disorder-induced localized states at the superconductor-insulator interface.…”

Spectroscopy of low-frequency noise and its temperature dependence in a superconducting qubit

“…Effective surface spins have recently been identified as one dominant source of low-frequency magnetic-flux noise 4,5 , detrimental to several types of superconducting qubits; however, open questions remain regarding the nature of these spins. Their noise is known to be due to local fluctuators [6][7][8][9] and the spectrum exhibits a 1/f α powerlaw dependence from hertz to tens of megahertz [10][11][12][13][14] .…”

Effect of magnetic Gd impurities on the superconducting state of amorphous Mo-Ge thin films with different thickness and morphology

“…The formation of localized magnetic moments in semiconductor heterostructures and devices are known to be carried by structural defects with unpaired electrons [1]. Localized magnetic moments were recently detected on the surface of a normal metal [2]; in superconducting systems, they are believed to be responsible for several unusual effects such as 1/f noise in SQUIDs and qubits [3] and an anomalous magnetic field enhancement of a critical current in nanowires [4]. The origin of spontaneously formed magnetic moments often remains unknown; on the other hand, their effects can be probed by magnetic moments that are introduced intentionally.…”