Hyperpolarized Molecular Nuclear Spins Achieve Magnetic Amplification

Abstract

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.

Figures (5)

• Figure 1: Magnetic amplification using hyperpolarized molecular nuclear spins. (a) Setup. The polarization system is designed to polarize molecular nuclear spins by bubbling $p$-H$_2$ gas through a neat liquid, and consists of a parahydrogen ($p$-$\text{H}_2$) generator, mass flow controller (MFC), valve (V) and back pressure regulator (BPR). The dipolar fields generated by the resulting hyperpolarized nuclear spins are detected using a $^{87}$Rb magnetometer, which includes a laser beam passed through a linear polarizer (LP), a quarter-wave plate ($\uplambda$/4) and a vapor cell, detected with a photodiode (PD) supp. (b) Schematic of magnetic amplification using proton spins from acetonitrile (1) and pyridine (2), as well as scalar-coupled spins from $^{15}$N-labeled acetonitrile (3). (c) Amplification signal using $^{15}$N-labeled acetonitrile.
• Figure 2: Demonstrations of magnetic amplification using proton spins. (a) Real-time response of proton spins to a near-resonant oscillating field. (b) Magnetic amplification using the hyperpolarized protons of acetonitrile (left) and pyridine (right). Colored curves show the spectral amplitude in the presence of hyperpolarized protons, while gray curves correspond to the oscillating field alone. The complete spectra are provided in the Supplemental Material supp. (c) Dependence of the ratio $\mathcal{R}$ on the amplitude of oscillating field.
• Figure 3: Anomalous frequency dependence of the proton amplification. (a) Dependence of the ratio $\mathcal{R}$ on the oscillating field frequency. (b) Phase difference $\varphi$ between the dipolar field and the oscillating field as a function of frequency. Colored lines indicate theoretical predictions. Insets illustrate the interference effect via vector diagrams of the dipolar field $B_{\text{s}}$ and the oscillating field $B_{\text{a}}$ in complex plane.
• Figure 4: Magnetic amplification using hyperpolarized scalar-coupled spins. (a) Zero-field NMR spectrum of $^{15}\text{N}$-acetonitrile, with a scalar coupling $J_{\text{NH}} \approx -1.688\,\text{Hz}$ between $^{15}\text{N}$ and $^1\text{H}$. (b) Amplification of the coupled spins near the $J_{\text{NH}}$ and $2J_{\text{NH}}$ frequencies. Colored curves show the spectral amplitude in the presence of hyperpolarized coupled spins, while gray curves correspond to the oscillating field alone. The corresponding transitions are shown in the inset. The complete spectra are provided in the Supplemental Material supp. (c) Dependence of the ratio $\mathcal{R}$ on the oscillating field frequency.
• Figure 5: Dependence of the amplification on hyperpolarization parameters. The $2J_\text{NH}$ manifold of the scalar-coupled spins is used as an example. (a) Dependence of the amplification $G$ on $p$-$\text{H}_2$ flow rate $Q$. The experimental data (black dots) are fitted using an empirical formula with parameters ${a \approx 0.001}$, ${b \approx 4.0}$ and ${Q_\text{c} \approx 151.7\,\text{sccm}}$ (solid line), where sccm denotes standard cubic centimeters per minute. (b) Dependence of $G$ on $p$-$\text{H}_2$ bubbling time. The data are fitted with fit parameters $c \approx 0.11$ and $T_1 \approx 21.1\,\text{s}$.