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arXiv:2610.14425

Hyperpolarization-enabled cooperative molecular magnetic amplification

Jingyan Xu, Raphael Kircher, Min Jiang, Xinhua Peng, Danila Barskiy, Dmitry Budker

quant-phSubmitted Oct 10, 2026• Announced Oct 13, 2026• 0 citations
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Abstract

Detecting weak magnetic fields at hertz frequencies is central to biomagnetism, geophysical sensing, and searches for new physics, yet it remains challenging because of technical systematics and environmental noise. Here, we realize a continuously operating molecular magnetic amplifier. Signal amplification by reversible exchange (SABRE) parahydrogen-based hyperpolarization maintains non-equilibrium spin order at zero field, turning scalar-coupled nuclear spins in liquid state molecules into an active, frequency-selective gain medium. Measurements of its complex magnetic susceptibility ($χ$) reveal an intrinsic response of $ | χ| \approx$ 10. By feeding the collectively generated magnetic field back to the spins below the threshold for self-sustained $J$-oscillation, we extend the effective spin coherence and cooperatively enhance the susceptibility up to $ | χ| \approx$ 80. Near threshold, this produces approximately fourfold amplification of a weak (20 pT) magnetic field at 3.37 Hz at the sensor location. These results establish hyperpolarized molecules as a tunable active magnetic medium for narrowband quantum sensing and precision measurement.

Comments
15 pages, 4 figures

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Detecting weak magnetic fields at hertz frequencies is central to biomagnetism, geophysical sensing, and searches for new physics, yet it remains challenging because of technical systematics and environmental noise. Here, we realize a continuously operating molecular magnetic amplifier. Signal amplification by reversible exchange (SABRE) parahydrogen-based hyperpolarization maintains non-equilibrium spin order at zero field, turning scalar-coupled nuclear spins in liquid state molecules into an active, frequency-selective gain medium. Measurements of its complex magnetic susceptibility (χχχ) reveal an intrinsic response of ∣χ∣≈ | χ| \approx∣χ∣≈ 10. By feeding the collectively generated magnetic field back to the spins below the threshold for self-sustained JJJ-oscillation, we extend the effective spin coherence and cooperatively enhance the susceptibility up to ∣χ∣≈ | χ| \approx∣χ∣≈ 80. Near threshold, this produces approximately fourfold amplification of a weak (20 pT) magnetic field at 3.37 Hz at the sensor location. These results establish hyperpolarized molecules as a tunable active magnetic medium for narrowband quantum sensing and precision measurement.
The use of nuclear spins as physical sensing systems is disadvantaged by their low signal responsivity, particularly when compared to sensing techniques based on electron spins. This primarily results from the small nuclear gyromagnetic ratio and the difficulties in achieving high spin polarization. Here we develop a new approach to investigating the response of hyperpolarized molecular nuclear spins to magnetic fields and demonstrate orders-of-magnitude enhanced magnetic responsivity over state-of-the-art proton and Overhauser magnetometers. Using hyperpolarized molecules with proton spins, we report the realization of magnetic amplification in linear and nonlinear types. We further extend this amplification to hyperpolarized scalar-coupled multi-spin molecules and observe substantial magnetic amplification exceeding 10%. Moreover, we observe an anomalous amplification with dispersive frequency dependence that originates from magnetic interference effects. Our work highlights the potential of hyperpolarized molecular nuclear spins for use in a new class of quantum sensors, with promising applications in both applied and fundamental physics, including highly accurate absolute magnetometry and the exploration of axion-nucleon exotic interactions.
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Quantum amplification is recognized as a key resource for precision measurements. However, most conventional paradigms employ an ensemble of independent particles that usually limit the performance of quantum amplification in gain, spectral linewidth, etc. Here we demonstrate a new signal amplification using cooperative 129Xe nuclear spins embedded within a feedback circuit, where the noble-gas spin coherence time is enhanced by at least one order of magnitude. Using such a technique, magnetic field can be substantially pre-enhanced by more than three orders and is in situ readout with an embedded 87Rb magnetometer. We realize an ultrahigh magnetic sensitivity of 4.0 fT/Hz1/2^{1/2}1/2 that surpasses the photon-shot noise and even below the spin-projection noise of the embedded atomic magnetometer, allowing for exciting applications including searches for dark matter with sensitivity well beyond supernova constraints. Our findings extend the physics of quantum amplification to cooperative spin systems and can be generalized to a wide variety of existing sensors, enabling a new class of cooperative quantum sensors.
Signal amplification by reversible exchange (SABRE) is a nuclear spin hyperpolarization technique in which the transient interaction of parahydrogen (pH2) and a target substrate with an iridium complex leads to polarization transfer to the substrate. Here, we use a parahydrogen-enhanced, spin-selective NMR method to investigate pyruvate binding, which is combined with exchange-model fitting and DFT calculations. Our study reveals several key findings that reshape the current understanding of SABRE: (a) intramolecular hydrogen exchange of the hydrides, occurring faster than pyruvate or H2 loss; (b) the discovery of a novel stable [Ir(H)2(IMes)(\k{appa}1-pyr)(DMSO)2] complex; and (c) the potential role of counterions (here Na+) in Ir-pyruvate binding. Previously unknown insights into complex kinetics and distributions as a function of temperature, [DMSO], [pyruvate], and hydrogen pressure are presented. The methods demonstrated here, exemplified by SABRE, provide a framework that is expected to guide future research in the field.
Quantum sensing is crucial for precision measurements, yet quantum sensor sensitivity is often limited by the coherence time of the quantum system. Here, we demonstrate a method to enhance coherence time through cooperative spins. Using a tunable feedback circuit, we induce cooperation among noble-gas ^{129}Xe spins, resulting in an impressive 18-fold coherence enhancement. Moreover, we show that the cooperative ^{129}Xe spins can significantly amplify magnetic signals by at least 3 orders of magnitude. Magnetic field sensing assisted with such a cooperative spin amplifier realizes the sensitivity of 4 fT/Hz^{1/2} and surpasses the spin-projection noise of the embedded ^{87}Rb spin gas magnetometer. These results pave the way for a new class of "cooperative quantum sensors," and open up exciting prospects in fundamental physics.
The use of atoms, molecules, and free electrons in quantum amplifiers has greatly advanced precision measurements, paving the way for the development of extremely-low-noise quantum devices such as masers and lasers. Here, we investigate the signal amplification of interacting spins and observe the amplification of magnetic fields using mixtures of interacting alkali-metal and noble gases. In contrast to noninteracting systems used as amplifiers, we demonstrate that interactions resulting from random atomic collisions give rise to two distinct amplification phenomena. These phenomena provide essential resources for enhancing quantum sensing capabilities. Our results show that magnetic fields can be amplified by at least two orders of magnitude, enhancing magnetic sensitivity to the femtotesla per root hertz level. Additionally, we report a counterpart phenomenon, deamplification, where the magnetic noise response is suppressed by at least one order of magnitude within certain frequency regimes. In this work alkali-metal and noble-gas spins are weakly coupled. We further explore how the performance of amplification changes with the interaction strength as the two spin gases gradually enter the strong-coupling regime, unveiling hitherto unexplored amplification effects that hold promise for enhancing precision measurements.