Chemical and structural identification of material defects in superconducting quantum circuits

Abstract

Quantum circuits show unprecedented sensitivity to external fluctuations compared to their classical counterparts, and it can take as little as a single atomic defect somewhere in a mm-sized area to completely spoil device performance. For improved device coherence it is thus essential to find ways to reduce the number of defects, thereby lowering the hardware threshold for achieving fault-tolerant large-scale error-corrected quantum computing. Given the evasive nature of these defects, the materials science required to understand them is at present in uncharted territories, and new techniques must be developed to bridge existing capabilities from materials science with the needs identified by the superconducting quantum circuit community. In this paper, we give an overview of methods for characterising the chemical and structural properties of defects in materials relevant for superconducting quantum circuits. We cover recent developments from in-operation techniques, where quantum circuits are used as probes of the defects themselves, to in situ analysis techniques and well-established ex situ materials analysis techniques. The latter is now increasingly explored by the quantum circuits community to correlate specific material properties with qubit performance. We highlight specific techniques which, given further development, look especially promising and will contribute towards a future toolbox of material analysis techniques for quantum.

Near-Field Localization of the Boson Peak on Tantalum Films for Superconducting Quantum Devices

“…5 It is therefore critical to devise methods for minimizing oxidation and/or controlling the chemical reactivity of metal surfaces found in quantum devices. 6,7 Tantalum has been identified as the leading material system for the fabrication of superconducting devices with state-ofthe-art performance, showing notable improvements over aluminum-or niobium-based devices. 8,9 While the initial results are promising, surface oxides remain the primary factor limiting device performance.…”

“…Qubit fidelity, and in turn the progress of quantum computing, is currently limited by decoherence due to the dissipative coupling of qubit modes to electric dipoles in amorphous materials and defect states. , These loss channels concentrate at amorphous oxides present at device interfaces, such as the metal–air interface, and their removal significantly improves the performance of planar superconducting devices in the low-temperature and low-power regimes . It is therefore critical to devise methods for minimizing oxidation and/or controlling the chemical reactivity of metal surfaces found in quantum devices. , …”

Evolution of $1/f$ Flux Noise in Superconducting Qubits with Weak Magnetic Fields

“…Several microscopic theories of magnetic defects in superconducting circuits with emergent 1/f flux noise spectra have been proposed [5,[12][13][14]. However, there is a lack of consensus in the community on both the nature and source of the spins and the spin physics which gives rise to the noise.…”

Near-field localization of the boson peak on tantalum films for superconducting quantum devices

“…These loss channels concentrate at amorphous oxides present at device interfaces, such as the metal-air interface [4], and their removal significantly improves the performance of planar superconducting devices in the low-temperature and low-power regimes [5]. It is therefore critical to devise methods to minimize oxidation and/or control the chemical reactivity of metal surfaces found in quantum devices [6,7].…”