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 1H, 129Xe and 6Li 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. Reducing the signal-to-noise ratio is a never-ending challenge for many types of experiments. Now, improved ratios are reported for nuclear magnetic resonance set-ups combining an external high-Q resonator and a low-Q input coil.