Magnetic Gradiometer for Detection of Zero- and Ultralow-Field Nuclear Magnetic Resonance

Magnetic sensors are important for detecting nuclear magnetization signals in nuclear magnetic resonance (NMR). As a complementary analysis tool to conventional high-field NMR, zero- and ultralow-field (ZULF) NMR detects nuclear magnetization signals in the sub-microtesla regime. Current ZULF NMR systems are always equipped with high-quality magnetic shieldings to ensure that ambient magnetic field noise does not dwarf the magnetization signal. An alternative approach is to separate the magnetization signal from the noise based on their differing spatial profiles, as can be achieved using a magnetic gradiometer. Here, we present a gradiometric ZULF NMR spectrometer with a magnetic gradient noise of 17 fT_{rms}{cm}^{-1}{Hz}^{-1/2} in the frequency range of 100-400 Hz, based on a single vapor cell (0.7x0.7x1.0{cm}^3). With applied white magnetic-field noise, we show that the gradiometric spectrometer achieves 13-fold enhancement in the signal-to-noise ratio (SNR) compared to the single-channel configuration. By reducing the influence of common-mode magnetic noise, this work enables the use of compact and low-cost magnetic shields. Gradiometric detection may also prove to be beneficial for eliminating systematic errors in ZULF-NMR experiments searching for exotic spin-dependent interactions and molecular parity violation.