Tunable resonators with high quality factors underpin the storage and retrieval of microwave-domain quantum information in spin ensembles used as long-lived quantum memories, and can enable multifrequency high-sensitivity electron spin resonance (ESR). The authors develop a single-layer technology, based on embedding nanoSQUIDs in superconducting niobium resonators, to realize high-quality frequency-tunable devices that are resilient to moderate magnetic fields. These devices will enable tunable-resonator-enhanced ESR protocols to be performed at specific fields and frequencies, such as storing quantum information in spins at low-decoherence ``clock transitions''.
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/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 Y2SiO5 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.