Direct Identification of Dilute Surface Spins on Al2O3 : Origin of Flux Noise in Quantum Circuits

Abstract

An on-chip electron spin resonance technique is applied to reveal the nature and origin of surface spins on Al_{2}O_{3}. We measure a spin density of 2.2×10^{17} spins/m^{2}, attributed to physisorbed atomic hydrogen and S=1/2 electron spin states on the surface. This is direct evidence for the nature of spins responsible for flux noise in quantum circuits, which has been an issue of interest for several decades. Our findings open up a new approach to the identification and controlled reduction of paramagnetic sources of noise and decoherence in superconducting quantum devices.

Resolving the positions of defects in superconducting quantum bits

“…Since these are limiting the performance of state-of-the-art circuits [10][11][12], further progress towards scaled-up quantum processors requires strong efforts to prevent the appearance of defects, e.g. by using better materials, improved fabrication procedures [13][14][15][16], and surface treatment to avoid contamination and parasitic adsorbates [17,18]. This endeavor needs to be guided by careful analysis of defect properties such as densities, electric dipole moments, and positions, in order to identify and improve the manufacturing steps that reduce defect formation, and to analyze the microscopic structure of defects.In this Letter, we present a method to extract information on the spatial positions of defects at the profile of the film edge in a qubit circuit.…”

Noise and loss of superconducting aluminium resonators at single photon energies

“…To have a stable locking of the Pound loop at such low energies, a parametric amplifier between the sample and the semiconductor amplifier, would be needed. We believe that this technique could lead to more efficient investigations of the origins of flux noise in superconducting circuits [31][32][33] .…”

Flux Noise in a Superconducting Transmission Line

“…• We have considered here a simplest geometry of the direct two-wire TL (similar to the TL with fixed ℓ used in [7] for characterization of spin defects at the interfaces). For a non-direct TL, the equation of motion in Sect IIA must be modified by taking into account any curvatures or angles along the TL.…”

“…A few additional mechanisms (noise due to dielectric losses, [23] fluctuations of charge at the Josephson junction, [24] interacting two-level defects, [7,25] and fluctuations of current in the TL caused by thermal generation of electron-hole pairs [26,27]) are possible for specific devices. The omitted mechanisms and the above-listed assumptions restrict the area of applicability of this paper but do not affect on the main result: the non-trivial spectral and size dependencies of the noise effect.…”

“…Superconducting circuits have been studied extensively for sensor applications at temperatures around 1 K, [6] where noise arises due to two-level defects. For operating temperatures of quantum hardware, near 10 −2 K, microscopic mechanisms of interaction with flux noises are under investigation now, see a detailed discussion [3] and recent experimental data [7,8] with references therein. In particular, mechanisms of low-frequency flux noise in TL have not been analyzed and their effects on quantum hardware, including spectral and size dependencies, have not been characterized.…”