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arXiv:2601.14321

High-Resolution Electron Paramagnetic Resonance

Colin J. Stephen, Anton Tcholakov, Maik Icker, Stuart M. Graham, Xiaoming Zhao, Robert Day, Jeanette Chattaway, T. John S. Dennis, Wolfgang Harneit, Gavin W. Morley

physics.ins-detphysics.chem-phquant-phSubmitted Jan 19, 2026• Announced Jan 21, 2026
arXivHTMLPDF

Abstract

Electron paramagnetic resonance (EPR) is a valuable tool for physics, chemistry, biology and medicine, providing complementary spectroscopic information to NMR. It has long been known that EPR at high magnetic fields offers greater spectral resolution, but limitations in the THz instrumentation have prevented the full realization of these opportunities. Here we describe an EPR spectrometer at the high magnetic field of 14 T using 396 GHz excitation, which adapts techniques from liquid-state NMR to obtain sharp EPR resonances with a width of 210 ppb (full-width half-maximum). We use this to measure resonance positions, and hence g-factors, with a precision that reaches $\pm$16 ppb. Our use of in-situ liquid-state NMR of our solvent within the same sample improves the accuracy of these measurements: it allows us to reference our EPR measurement back to dilute gas 3He NMR for which quantum calculations are accurate. We measure the g-factor of N@C60 in deuterated toluene as g = 2.002 099 09 (3), where the 3 in brackets means that the uncertainty on the last digit is $\pm$3.

Comments
15 pages, 11 figures

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Electron paramagnetic resonance (EPR) is a valuable tool for physics, chemistry, biology and medicine, providing complementary spectroscopic information to NMR. It has long been known that EPR at high magnetic fields offers greater spectral resolution, but limitations in the THz instrumentation have prevented the full realization of these opportunities. Here we describe an EPR spectrometer at the high magnetic field of 14 T using 396 GHz excitation, which adapts techniques from liquid-state NMR to obtain sharp EPR resonances with a width of 210 ppb (full-width half-maximum). We use this to measure resonance positions, and hence g-factors, with a precision that reaches ±\pm±16 ppb. Our use of in-situ liquid-state NMR of our solvent within the same sample improves the accuracy of these measurements: it allows us to reference our EPR measurement back to dilute gas 3He NMR for which quantum calculations are accurate. We measure the g-factor of N@C60 in deuterated toluene as g = 2.002 099 09 (3), where the 3 in brackets means that the uncertainty on the last digit is ±\pm±3.
Abstract Electron paramagnetic resonance (EPR) spectroscopy is an important technology in physics, chemistry, materials science, and biology. Sensitive detection with a small sample volume is a key objective in these areas, because it is crucial, for example, for the readout of a highly packed spin based quantum memory or the detection of transition metals in biomaterials. Here, we demonstrate a novel EPR spectrometer using a single artificial atom as a sensitive detector of spin magnetization. The artificial atom, a superconducting flux qubit, provides advantages in terms of its strong coupling with magnetic fields. We estimate a sensitivity of ~400 spins·Hz−1/2 with a magnetic sensing volume around 10−14λ3 (50 femtoliters), where λ is the wavelength of the irradiated microwave. Our artificial atom works as a highly sensitive EPR spectrometer with micron-scale area with future opportunity for measuring single spins on the nanometer scale.
Modern experimental techniques can generate magnetic fields of the form H(t) = H0 z-hat + H1 [x-hat cos(ωt) + y-hat sin(ωt)], at frequencies within an order of magnitude of the nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) frequencies, ωn0 and ωe0, respectively, when acting on atoms or molecules. We derive simple closed-form expressions for the exact nuclear- and electronic-spin wave functions that enable controlled transitions between entangled states, allowing an atom or molecule to function as a quantum computer. These solutions also enable precise NMR or EPR measurements of nuclear moments in atoms and molecules. We present examples relevant to measurements of the nuclear moments of 14N, 7Li, and 133Cs. Because existing hyperfine measurements of the lowest three nuclear moments of 133Cs are mutually inconsistent, the proposed NMR/EPR experiments provide a route to measuring all seven of its nuclear moments with high precision.
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 ability to measure and reduce systematic errors in single-qubit logic gates is crucial when evaluating quantum computing implementations. We describe pulsed electron paramagnetic resonance (EPR) sequences that can be used to measure precisely even small systematic errors in rotations of electron-spin-based qubits. Using these sequences we obtain values for errors in the rotation angle and axis for single-qubit rotations using a commercial EPR spectrometer. We conclude that errors in qubit operations by pulsed EPR are not limiting factors in the implementation of electron-spin-based quantum computers.
Quantum systems composed of solid-state electronic spins can be sensitive detectors of narrowband magnetic fields. A prominent example is the nitrogen-vacancy (NV) center in diamond, which has been employed for magnetic spectroscopy with high spatial and spectral resolution. However, NV-diamond spectroscopy protocols are typically based on dynamical decoupling sequences, which are limited to low-frequency signals ( ≲ 20 MHz) due to the technical requirements on microwave (MW) pulses used to manipulate NV electronic spins. In this work, we experimentally demonstrate a high-resolution magnetic spectroscopy protocol that integrates a quantum frequency mixing (QFM) effect in a dense NV ensemble with coherently averaged synchronized readout (CASR) to provide both a wide range of signal frequency detection and sub-Hz spectral resolution. We assess the sensitivity of this QFM-CASR protocol across a frequency range of 10 MHz to 4 GHz. By measuring the spectra of multi-frequency signals near 0.6, 2.4 and 4 GHz, we demonstrate sub-Hz spectral resolution with a nT-scale noise floor for the target signal, and precise phase measurement with error < 1 ∘ . Compared to state-of-the-art NV-diamond techniques for high-resolution magnetic spectroscopy, the QFM-CASR protocol extends the signal frequency range by more than an order of magnitude, enabling applications in radio frequency (RF) and MW signal microscopy and analysis, as well as tesla-scale nuclear magnetic resonance (NMR) spectroscopy of small samples.
Nitrogen vacancy centers in diamond (NVs) are point defects well suited to quantum sensing of magnetic fields. NV spin states can be initialized and read out optically with high spatial resolution, enabling detection of pL-scale volumes of nuclear spins. Using Coherently Averaged Synchronized Readout (CASR), standard NMR spectroscopy techniques such as free decay, spin echo, and PGSE have been performed with Hz-scale spectral resolution using NV-diamond sensors. However, using NV detection for conventional high-resolution NMR at multi-tesla fields is challenging due to the need to manipulate NV electronic spins at resonance frequencies 100 GHz or higher. Low-field J-coupling spectroscopy provides an alternative path to chemical resolution in a regime more easily accessible to NVs. Here, we demonstrate homonuclear and heteronuclear J-coupling spectroscopy using an NV-NMR spectrometer based on permanent magnets operating at $\sim$890 kHz proton Larmor frequency (bias magnetic field $\sim21mT).ThisfieldstrengthenableschemicalcharacterizationbasedonJ−couplingusingsequencessuchasSynchronizedEcho(SyncE)andSpin−EchoDoubleResonance(SEDOR),whichdonotrequirethestrongchemicalshiftdispersionandpristinefieldhomogeneityofhigh−fieldNMR.ThefrequencyrangeiseasilydetectedbyNVsusingstandardXYdynamicaldecouplingsequencesandiswell−suitedforapplyingOverhauserDNP,whichcanboostprotonsignalbytwoordersofmagnitude.WeacquireinitialNV−NMRmeasurementsofT21 mT). This field strength enables chemical characterization based on J-coupling using sequences such as Synchronized Echo (SyncE) and Spin-Echo Double Resonance (SEDOR), which do not require the strong chemical shift dispersion and pristine field homogeneity of high-field NMR. The frequency range is easily detected by NVs using standard XY dynamical decoupling sequences and is well-suited for applying Overhauser DNP, which can boost proton signal by two orders of magnitude. We acquire initial NV-NMR measurements of T21mT).ThisfieldstrengthenableschemicalcharacterizationbasedonJ−couplingusingsequencessuchasSynchronizedEcho(SyncE)andSpin−EchoDoubleResonance(SEDOR),whichdonotrequirethestrongchemicalshiftdispersionandpristinefieldhomogeneityofhigh−fieldNMR.ThefrequencyrangeiseasilydetectedbyNVsusingstandardXYdynamicaldecouplingsequencesandiswell−suitedforapplyingOverhauserDNP,whichcanboostprotonsignalbytwoordersofmagnitude.WeacquireinitialNV−NMRmeasurementsofT_1,T, T,T_2,andOverhauserDNPinglycerolbeforeadvancingtoSEDORspectroscopyofmethanol−, and Overhauser DNP in glycerol before advancing to SEDOR spectroscopy of methanol-,andOverhauserDNPinglycerolbeforeadvancingtoSEDORspectroscopyofmethanol−^{13}Candamethanol−C and a methanol-Candamethanol−^{13}C/acetonitrile−2−C/acetonitrile-2-C/acetonitrile−2−^{13}$C mixture at sample volumes $\sim$50 pL, in which we demonstrate measurement of 1JCH^1J_\text{CH}1JCH​ couplings and resolution between species. We also acquire a homonuclear J-coupling spectrum of a natural-abundance propylene carbonate sample.