  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.
