External high-quality-factor resonator tunes up nuclear magnetic resonance

The development of powerful sensors for the detection of weak electromagnetic fields is crucial for many spectroscopic applications, in particular for nuclear magnetic resonance (NMR). Here, we present a comprehensive theoretical model for boosting the NMR signal-to-noise ratio, validated by liquid-state H-1, Xe-129 and Li-6 NMR experiments at low frequencies, using an external resonator with a high quality-factor combined with a low-quality-factor input coil. In addition to an enhanced signal-to-noise ratio, this approach exhibits striking features such as a high degree of flexibility with respect to input coil parameters and a square-root dependence on the sample volume, and signifies an important step towards compact NMR spectroscopy at low frequencies with small and large coils.