Developing new materials for superconducting quantum circuits

_{Interactions between oxygen and the surface of Al2O3 can result in electron transport to the outer sphere and radical production. Credit: National Physical Laboratory}

NPL scientists, in partnership with specialists in physical chemistry, have used cutting-edge electron paramagnetic resonance (EPR) techniques to get a better understanding of materials crucial to superconducting quantum circuits. Their findings were recently published in Science Advances.

The size and complexity of superconducting quantum computers have expanded dramatically in recent years, and the focus is now on demonstrating fault-tolerant error-corrected quantum computing. At the moment, progress is being stymied by the comparatively short coherence time and state integrity of qubits. The majority of these impediments are a result of atomic-scale material imperfections interacting with the qubits. Due to their quantum nature, the origins of these defects are notoriously difficult to investigate: they only manifest at energy scales and circumstances relevant to qubits and are so far mainly inaccessible using tools available to material scientists.

The team was able to investigate a specific surface radical on Al2O3 (a material found in all current superconducting quantum computers) in detail by combining high magnetic field EPR and nuclear spectroscopy methods.

The investigation uncovered a complicated structure for the radical: an electron coupled to many Al atoms in the Al2O3 lattice, as well as numerous individual hydrogen nuclei. This enabled the radical to be explicitly attributed to a surface imperfection. This is the first experimental investigation to demonstrate the precise structure and chemistry of such surface imperfections associated with decoherence in superconducting quantum circuits.

Now that we have a complete understanding of the particular aspects of this flaw, we can begin considering approaches to mute it. Silencing, rather than eradicating, this specific flaw looks to be the most plausible path forward for future devices with more coherence, as this defect is now acknowledged to be inherent to the chemistry naturally happening at device surfaces.

The work makes a significant contribution to the area of materials for quantum circuits by providing one of the first direct ways to chemical and structural fault detection. Until now, the area has been primarily reliant on materials science, which operates at fundamentally different energy scales and defect concentrations. These approaches are capable of revealing faults but do not provide a direct link to defects that emerge and interact with quantum circuitry. Thus, we urgently want new materials science that can comprehend faults as they manifest themselves in quantum circuits without requiring lengthy, and frequently inconclusive, correlation studies between materials, manufacturing techniques, and device performance. The strategy described in this article is one of the first to take a direct approach to this problem.

Senior research scientist at NPL, Sebastian de Graaf, stated: “We hope that our discovery will inspire material scientists and chemists worldwide to develop and enhance comparable methodologies for studying the materials used in solid state quantum circuits. We may now study the influence of a broad variety of chemical treatments in a straightforward manner with the objective of identifying a technique that minimises the number of discovered flaws.”

Further information: Sun Un et al, On the nature of decoherence in quantum circuits: Revealing the structural motif of the surface radicals in α-Al2O3, Science Advances (2022). DOI: 10.1126/sciadv.abm6169

Journal information: Science Advances

Source: National Physical Laboratory