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IEEE International Conference on Quantum Computing and Engineering (QCE)

Detection of Surface Spins Causing Flux Noise in Quantum Devices Using Electron Spin Resonance

Dustin T. Hassenmayer, Patrick M. Lenahan, Jason Thomas Ryan, Stephen J. Moxim

Published Aug 30, 2025• 0 citations

Abstract

The improvement of quantum computing technologies relies on the reduction of flux noise that contributes to decoherence. Recent work has shown that the majority of flux noise is due to impurities and defects on the surface of the qubits [1], [2]. Electron spin resonance (ESR) is a prime candidate to detect these defects, but lacks the sensitivity and ability to provide information on the physical location of the defects. We report upon ESR measurements utilizing a non-resonant probe integrated within a semiconductor wafer probing station which could be used to detect paramagnetic defects believed to contribute to flux noise in quantum devices. Our approach greatly enhances ESR's sensitivity and provides a high spatial resolution on the surface of unaltered fully processed wafers and devices at room temperature. We demonstrate the effectiveness of our method to detect defects on the surface of materials with a spin sensitivity improvement of roughly $10,000 × compared to classical ESR. The integration of a non-resonant probe into a standard semiconductor wafer probing setup has already been shown to be able to obtain a 2D map of unpaired spins on the surface of samples [3]. The technique offers a pathway for characterization of the defects which contribute to flux noise, which would provide critical insight into reducing decoherence mechanisms.

DOI: 10.1109/qce65121.2025.10429

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The improvement of quantum computing technologies relies on the reduction of flux noise that contributes to decoherence. Recent work has shown that the majority of flux noise is due to impurities and defects on the surface of the qubits [1], [2]. Electron spin resonance (ESR) is a prime candidate to detect these defects, but lacks the sensitivity and ability to provide information on the physical location of the defects. We report upon ESR measurements utilizing a non-resonant probe integrated within a semiconductor wafer probing station which could be used to detect paramagnetic defects believed to contribute to flux noise in quantum devices. Our approach greatly enhances ESR's sensitivity and provides a high spatial resolution on the surface of unaltered fully processed wafers and devices at room temperature. We demonstrate the effectiveness of our method to detect defects on the surface of materials with a spin sensitivity improvement of roughly $10,000 × compared to classical ESR. The integration of a non-resonant probe into a standard semiconductor wafer probing setup has already been shown to be able to obtain a 2D map of unpaired spins on the surface of samples [3]. The technique offers a pathway for characterization of the defects which contribute to flux noise, which would provide critical insight into reducing decoherence mechanisms.

Superconducting circuits are promising components for future quantum information technologies such as quantum computing, quantum simulation, and quantum sensing. Superconducting quantum interference devices (SQUIDs) and superconducting flux qubits can be used as a sensitive magnetic field sensor to detect local electron spin resonance (ESR). These types of sensor are characterized by good sensitivity and high spatial resolution for electron spins. In fact, we have achieved a sensitivity of 20 spins/

\sqrt{\text {Hz}}

with a sensing volume of 6 fl combining a flux qubit and qubit readout using a Josephson bifurcation amplifier. As a demonstration, we measured the ESR spectrum of erbium electron spins in a Y

_{2}

SiO

_{5}

crystal and nitrogen vacancy centers in diamond. In both cases, we obtained reasonable electron spin parameters by fitting a simulation to the spectrum. In this way, we proved superconducting circuits to be promising magnetic field sensors for evaluating the properties of solid materials.

We use a superconducting flux qubit for the sensing application of electron spin resonance (ESR) spectroscopy.The superconducting flux qubit is a highly sensitive magnetometer with micrometer-scale spatial resolution.The sensitivity of our spectrometer is 400 spins/√Hz, and the sensing volume is 0.05 pL.This high sensitivity and high spatial resolution enable us to investigate a small number of electron spins in a microscale area.The spectrometer also obtains a wider ESR spectrum range with twoparameter (frequency and magnetic field) scanning.This enables us to refine the material parameters.

Superconducting Quantum Interference Devices (SQUIDs) are currently being used as flux qubits and read-out detectors in a variety of solid-state quantum computer architectures. The main limitation of SQUID qubits is that they have a coherence time of the order of 10 us, due to the presence of intrinsic flux noise that is not yet fully understood. The origin of flux noise is currently believed to be related to spin impurities present in the materials and interfaces that form the device. Here we present a novel numerical method that enables calculations of the flux produced by spin impurities even when they are located quite close to the SQUID wire. We show that the SQUID will be particularly sensitive to spins located at its wire edges, generating flux shifts of up to 4 nano flux quanta, much higher than previous calculations based on the software package FastHenry. This shows that spin impurities in a particular region along the wire's surface play a much more important role in producing flux noise than other spin impurities located elsewhere in the device.

The nitrogen-vacancy (NV) defect center in diamond has demonstrated great capability for nanoscale magnetic sensing and imaging for both static and periodically modulated target fields. However, it remains a challenge to detect and image randomly fluctuating magnetic fields. Recent theoretical and numerical works have outlined detection schemes that exploit changes in decoherence of the detector spin as a sensitive measure for fluctuating fields. Here we experimentally monitor the decoherence of a scanning NV center in order to image the fluctuating magnetic fields from paramagnetic impurities on an underlying diamond surface. We detect a signal corresponding to roughly 800

μ_B

in 2 s of integration time, without any control on the target spins, and obtain magnetic-field spectral information using dynamical decoupling techniques. The extracted spatial and temporal properties of the surface paramagnetic impurities provide insight to prolonging the coherence of near-surface qubits for quantum information and metrology applications.

Solid-state devices used for quantum computation and quantum sensing applications are adversely affected by loss and noise caused by spurious, charged two-level systems (TLS) and stray paramagnetic spins. These two sources of noise are interconnected, exacerbating the impact on circuit performance. We use an on-chip electron spin resonance (ESR) technique, with niobium nitride ( Nb N ) superconducting resonators, to study surface spins on silicon and the effect of postfabrication surface treatments. We identify two distinct spin species that are characterized by different spin-relaxation times and respond selectively to various surface treatments (annealing and hydrofluoric acid). Only one of the two spin species has a significant impact on the TLS-limited resonator quality factor at low-power (near-single-photon) excitation. We observe a three- to fivefold reduction in the total density of spins after surface treatments and demonstrate the efficacy of ESR spectroscopy in developing strategies to mitigate loss and decoherence in quantum systems. Published by the American Physical Society 2024

The nanoscale charge environment critically influences the performance of semiconductor devices, yet traditional characterization techniques usually lack the spatial resolution to resolve nanoscale charge heterogeneity and identify microscopic noise sources. Here, we address this critical gap by utilizing individual PL5 color centers in commercial silicon carbide as room-temperature, broadband (near-DC to GHz) quantum spectrometers. We report the first real-time, nanoscale observation of single-charge tunneling dynamics at room temperature in 4H-SiC by monitoring the random telegraph noise using optically detected magnetic resonance (ODMR). This capability enables an electrical noise imaging technique that shows distinct noise variations on different wafers. Employing dynamical decoupling and

T_1

relaxation spectroscopy, we observe correlations of noise spectral density across frequency bands and obtain the nanoscale electron paramagnetic resonance (EPR) fingerprint of spin defects in SiC. Ultimately, through surface chemical engineering, we identify the native oxide layer as the main source of resolved electromagnetic fluctuations. This work establishes a versatile atomic-scale metrology platform bridging quantum sensing and semiconductor engineering, offering critical insights for optimizing third-generation semiconductor interfaces and advancing solid-state quantum technologies.