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Micro-ESR is a powerful method for studying the noise and decoherence in solid state quantum circuits

A collaboration between the National Physical Laboratory (NPL), UK, Chalmers University of Technology, Microtechnology and Nanoscience, Sweden, and the Laboratoire de Physique Theorique et Hautes Energies (LPTHE), France, has demonstrated that removing surface spin defects dramatically reduces the charge noise of quantum devices such as sensors and computers.

Superconducting quantum devices such as qubits and sensors are plagued by performance-degrading material defects which come with many different properties. Typically, these defects can be divided into two types: surface spins that cause magnetic flux noise and local variations in the magnetic field, and surface charges that cause charge (or dielectric) noise. Traditionally, the two types of defects and the noise they produce have been studied separately. The breakthrough result, described in a paper in Nature Communications, is that the sources of charge noise have been identified by their spin signature. In doing so, the researchers have shown that the removal of surface spin defects dramatically reduces the charge noise.

The technique used in this study, micro-ESR, is an on-chip version of the well-established electron spin resonance (ESR) spectroscopy technique, many orders of magnitude more sensitive when it comes to surface defects. In a 2017 paper published in Physical Review Letters, the team identified the presence of very dilute surface spins in quantum devices. In the new study, the authors used a planar superconducting microwave resonator as a sensitive probe for both micro-ESR and charge noise. They showed that the removal of some of the surface spins also, surprisingly, suppress the charge noise tenfold. This experiment shows that micro-ESR is a very powerful tool as it can now be used to identify the “chemical fingerprint” of both sources of flux noise and sources of charge noise in quantum devices.

Sebastian de Graaf, Senior Researcher at NPL and lead scientist of the study said: “The important step demonstrated in this study is that the same technique can be used to reveal the origin of flux noise and the chemical fingerprint of the sources of charge noise. This makes micro-ESR a truly invaluable metrological tool in the development of high-coherence materials and for the scale-up of quantum computing circuits.”

The major contribution of the Swedish group at the department of microtechnology and nanoscience at Chalmers University of Technology was the development of magnetic-field resilient superconducting resonators at the heart of micro-ESR. The French collaborators at LPTHE contributed their theoretical expertise in noise-generating defects, which explain the findings. While this particular study looked at a specific material, sapphire, commonly used in the fabrication of superconducting quantum circuits, the team now hopes to apply the same methodology to a much wider range of materials that are relevant for the emerging quantum industry.

Tobias Lindström, Senior Research Scientist at NPL and co-author of the study said: “Quantum devices are inherently extremely sensitive to their environment and existing tools simply do not have the sensitivity needed to fully characterise the materials we use to fabricate qubits. New tools and techniques are therefore very much needed if we want to, for example, build a practical quantum computer.”

Dr Andrey Danilov, Senior Research Fellow at Chalmers University of Technology, Microtechnology and Nanoscience concludes: “Ultimately we want to understand and eliminate these types of defects in all materials used in solid state quantum technologies, as this is a limiting factor for a wide range of technologies. Here we demonstrate a very important step in this direction by combining a range of advanced measurement techniques to demonstrate a new method for material characterisation at the quantum level, where conventional analytical techniques are not applicable.”

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