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Superconducting circuits for characterizing spins and charge defects with microsecond coherence

Matteo Boselli

Published Dec 11, 2025• 0 citations
Publisher

Abstract

The detection of electric and magnetic dipole ensembles is a fundamental tool across physics, chemistry, and biology. Here, we combine low-loss microwave superconducting circuits with advanced spectroscopic techniques to probe paramagnetic samples and dielectric defects at millikelvin temperatures. In this regime, full polarization, quantum-limited amplifiers, and superconducting resonators with narrow linewidths enable a dramatic boost in sensitivity. Such capabilities even allow the study of individual dielectric defects within the circuit materials themselves. In the context of electron spin resonance (ESR), hybrid spin-superconducting platforms have already reached single-spin detection and sub-picoliter sample volumes.In our first experiment, we observed—for the first time—the emission of Hahn echoes from dielectric defects in a Josephson Traveling-Wave Parametric Amplifier, a widely used quantum-limited amplifier. These echoes, triggered at drive powers close to the amplifier's saturation threshold, can degrade device performance. Using standard pulse sequences, we characterized their dynamics, allowing to estimate the defect density in the device dielectric layers and to quantify their impact on amplifier operation.In a second experiment, we addressed the challenge of extending ESR detection to arbitrary spin ensembles, which may be highly disordered and exhibit short coherence times. While high-quality factor superconducting resonators offer exceptional detection sensitivity for spins, their long ring-down times hinder the study of rapidly dephasing spins. To overcome this, we developed a dynamically tunable resonator based on the kinetic inductance of Niobium-Titanium-Nitride (NbTiN), a superconducting alloy resilient to magnetic fields. Using fast DC current pulses, we tuned the resonator linewidth on the microsecond timescale while preserving their performance for ESR detection. We benchmarked the spectrometer by performing time-resolved spectroscopy on a 20-picoliter sample of α,γ-bisdiphenylene-β-phenylallyl at millikelvin temperatures, achieving a sensitivity only one order of magnitude below state-of-the-art experiments on highly coherent ensembles.Our results establish a versatile ESR platform operating at millikelvin temperatures, with potential applications ranging from materials characterization to the detection of unpaired electrons in single biological cells.

DOI
10.70675/6fd0a2f9z8442z4047zb4cbz1788c1aa31b1

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•Jan 1, 2016•0 citations
The detection of electric and magnetic dipole ensembles is a fundamental tool across physics, chemistry, and biology. Here, we combine low-loss microwave superconducting circuits with advanced spectroscopic techniques to probe paramagnetic samples and dielectric defects at millikelvin temperatures. In this regime, full polarization, quantum-limited amplifiers, and superconducting resonators with narrow linewidths enable a dramatic boost in sensitivity. Such capabilities even allow the study of individual dielectric defects within the circuit materials themselves. In the context of electron spin resonance (ESR), hybrid spin-superconducting platforms have already reached single-spin detection and sub-picoliter sample volumes.In our first experiment, we observed—for the first time—the emission of Hahn echoes from dielectric defects in a Josephson Traveling-Wave Parametric Amplifier, a widely used quantum-limited amplifier. These echoes, triggered at drive powers close to the amplifier's saturation threshold, can degrade device performance. Using standard pulse sequences, we characterized their dynamics, allowing to estimate the defect density in the device dielectric layers and to quantify their impact on amplifier operation.In a second experiment, we addressed the challenge of extending ESR detection to arbitrary spin ensembles, which may be highly disordered and exhibit short coherence times. While high-quality factor superconducting resonators offer exceptional detection sensitivity for spins, their long ring-down times hinder the study of rapidly dephasing spins. To overcome this, we developed a dynamically tunable resonator based on the kinetic inductance of Niobium-Titanium-Nitride (NbTiN), a superconducting alloy resilient to magnetic fields. Using fast DC current pulses, we tuned the resonator linewidth on the microsecond timescale while preserving their performance for ESR detection. We benchmarked the spectrometer by performing time-resolved spectroscopy on a 20-picoliter sample of α,γ-bisdiphenylene-β-phenylallyl at millikelvin temperatures, achieving a sensitivity only one order of magnitude below state-of-the-art experiments on highly coherent ensembles.Our results establish a versatile ESR platform operating at millikelvin temperatures, with potential applications ranging from materials characterization to the detection of unpaired electrons in single biological cells.
Electron spin resonance (ESR) is a powerful spectroscopy method which allows to identify and characterize paramagnetic species. It usually relies on detecting microwave radiation absorbed or emitted by the spins into a microwave resonator tuned to their Larmor precession frequency. Because of the weak spin-microwave coupling, conventional ESR spectroscopy has a low sensitivity. Recent experiments have demonstrated that superconducting quantum circuits have the potential to enhance the spin detection sensitivity to single spin detection; by namely using high quality factor small mode volume resonator. However, these demonstrations have been realized in very restrictive conditions, using well-known spin systems. To probe samples coming from chemistry or biology, quantum-circuit enabled spectrometers have to be adapted to be robust to losses. They have typically two origins: the dielectric losses introduced by the samples, mostly aqueous, and losses coming from the strong magnetic field required to tune the electronic spins to the resonator frequency. This PhD work centered on implementing a resonator that would achieve the same ESR sensitivity while being robust to dielectric losses and strong magnetic fields. Using NbTiN films deposited on sapphire, microwave resonators were shown to have quality factors above Q=105 in magnetic field, even when samples with dielectric losses above tan d > 0.05 were introduced. A study of the resonator intrinsic losses showed marked drops at multiple specific magnetic fields: a signature of coupling to spurious systems. Over the course of the PhD, mitigation nanofabrication techniques for these contaminants were developed, such as hard mask etching and annealing, to remove all but one of this contaminants. Finally, test of the spectrometer on a benchmark species (BDPA, α,γ-Bisdiphenylene-β-phenylallyl, a known ESR marker) were realized.
The use of superconducting microresonators together with quantum-limited Josephson parametric amplifiers has enhanced the sensitivity of pulsed electron spin resonance (ESR) measurements by more than four orders of magnitude. So far, the microwave resonators and amplifiers have been designed as separate components due to the incompatibility of Josephson junction–based devices with magnetic fields. This has produced complex spectrometers and raised technical barriers toward adoption of the technique. Here, we circumvent this issue by coupling an ensemble of spins directly to a weakly nonlinear and magnetic field–resilient superconducting microwave resonator. We perform pulsed ESR measurements with a 1-pL mode volume containing 6 × 10 7 spins and amplify the resulting signals within the device. When considering only those spins that contribute to the detected signals, we find a sensitivity of 2.8 × 1 0 3 spins / Hz for a Hahn echo sequence at a temperature of 400 mK. In situ amplification is demonstrated at fields up to 254 mT, highlighting the technique’s potential for application under conventional ESR operating conditions.
The use of superconducting micro-resonators in combination with quantum-limited Josephson parametric amplifiers has in recent years lead to more than four orders of magnitude improvement in the sensitivity of pulsed Electron Spin Resonance (ESR) measurements. So far, the microwave resonators and amplifiers have been designed as separate components, largely due to the incompatibility of Josephson junction-based devices with even moderate magnetic fields. This has led to complex spectrometers that operate under strict environments, creating technical barriers for the widespread adoption of the technique. Here we circumvent this issue by inductively coupling an ensemble of spins directly to a weakly nonlinear microwave resonator, which is engineered from a magnetic field-resilient thin superconducting film. We perform pulsed ESR measurements with a 111~pL effective mode volume and amplify the resulting spin signal using the same device, ultimately achieving a sensitivity of 2.8×1032.8 \times 10^32.8×103 spins in a single-shot Hahn echo measurement at a temperature of 400 mK. We demonstrate the combined functionalities at fields as large as 254~mT, highlighting the technique's potential for application under more conventional ESR operating conditions.
Caractérisation de spins et de défauts diélectriques d'une cohérence de l'ordre de la microseconde avec des circuits supraconducteurs La détection d'ensembles de dipôles électriques et magnétiques est un outil fondamental en physique, chimie et biologie. Dans cette thèse, nous combinons des circuits supraconducteurs micro-ondes à faibles pertes avec des techniques spectroscopiques avancées pour sonder des échantillons paramagnétiques et des défauts diélectriques à des températures de l'ordre du millikelvin. Dans ce régime, la polarisation entière, les amplificateurs à la limite quantique et les résonateurs supraconducteurs à largeur de raie étroite permettent d'augmenter considérablement la sensibilité. Ces capacités permettent même d'étudier des défauts diélectriques individuels au sein même des matériaux des circuits. Dans le contexte de la résonance de spin électronique (ESR), les plateformes hybrides spin-supraconductrices ont déjà atteint la détection de spin unique et des volumes d'échantillons inférieurs au picolitre. Dans une première expérience, nous avons observé pour la première fois l'émission d'échos de Hahn provenant de défauts diélectriques dans un amplificateur paramétrique à onde progressive de Josephson, un amplificateur quantique largement utilisé. Ces échos, émis pour des puissances d'impulsions de contrôle proches du seuil de saturation de l'amplificateur, peuvent dégrader les performances du dispositif. À l'aide de séquences d'impulsions standard, nous avons caractérisé leur dynamique, ce qui nous a permis d'estimer la densité des défauts dans les couches diélectriques du dispositif et de quantifier leur impact sur le fonctionnement de l'amplificateur.Dans une deuxième expérience, nous avons relevé le défi d'étendre la détection ESR de haute sensibilité à des ensembles de spins arbitraires, qui peuvent être très désordonnés et présenter des temps de cohérence courts. Alors que les résonateurs supraconducteurs à facteur de qualité élevé offrent une sensibilité exceptionnelle, leurs longs temps de décroissance empêchent l'étude des spins à déphasage rapide. Pour surmonter ce problème, nous avons développé un résonateur à accord dynamique basé sur l'inductance cinétique du nitrure de niobium-titane (NbTiN), un alliage supraconducteur résistant aux champs magnétiques. À l'aide d'impulsions de courant continu rapides, nous avons accordé la largeur de raie du résonateur à l'échelle de la microseconde tout en préservant ses performances ESR. Nous avons évalué le spectromètre en effectuant une spectroscopie résolue en temps sur un échantillon de 20 picolitres d'α,γ-bisdiphénylène-β-phénylallyle à des températures de l'ordre du millikelvin, obtenant une sensibilité inférieure d'un seul ordre de grandeur à celle des expériences de pointe sur des ensembles hautement cohérents.Nos résultats établissent une plateforme ESR polyvalente fonctionnant à des températures de l'ordre du millikelvin, avec des applications potentielles allant de la caractérisation des matériaux à la détection d'électrons non appariés dans des cellules biologiques individuelles.
In the field of circuit quantum electrodynamics (c-QED), the coherent interaction of two-level systems (TLSs) with photons, confined in a superconducting microwave\nresonator, opens up new possibilities for quantum computing experiments.\nThis thesis contributes to the expansion of c-QED tool-box with slow propagation line-based solutions for ubiquitous techniques, such as cryogenic Near Field Scanning Microwave Microscopy (NSMM), Electron Spin Resonance (ESR) spectroscopy and Traveling Wave Parametric Amplifier (TWPA). For NSMM, novel compact superconducting fractal resonators have been developed to be directly integrated on a scanning probe. We report NSMM operation based on a microwave high-Q resonator populated with less than 10^3 photons and demonstrate a capacitive sensitivity of 0.38 aF/rtHz.\nThe unique properties and the design flexibility of fractal resonators also boost their resiliency to strong magnetic fields for ESR studies. The reported high Q-factors above 10^5 in a magnetic field up to 0.4 T translate into ESR sensitivity of 5 10^5 spins/rtHz.\nFurthermore, we demonstrate the operation of a practical TWPA based on a slow propagation fractal line. We achieve per unit length gain of > 0.5 dB/cm and total gain of ~6 dB for a 10 cm long line. Due to a radically shortened line, the amplifier is less vulnerable to fabrication defects. Moreover, due to a successful impedance matching\nbetween the amplifier line and in/out terminals, the obtained gain vs frequency characteristic has only moderate ripples. To mitigate a common TWPA problem of\ncoupling to parasitic ground plane resonances, we deploy a novel multilayer fabrication technology, which combines high and low kinetic inductance (KI) elements.\nFinally, we present an alternative implementation of a slow propagation line: a microstrip line with a thin film Atomic Layer Deposition (ALD) Al2O3 oxide. The resonator, based on a segment of a microstrip line, has a Q-factor on the order of 10^4 at single photon powers, reaching up to 10^5 at higher powers. As an additional functionality,\nwe incorporate dc current control over KI so that the resonance frequency is tuned by 62 MHz range, which corresponds to a KI-related nonlinearity of 3%.