Para-hydrogen raser delivers sub-millihertz resolution in nuclear magnetic resonance

A method for narrowing the NMR linewidth of specific molecules to the sub-millihertz range—two orders of magnitude below the natural linewidth—could open up new avenues for molecular characterization. The precision of nuclear magnetic resonance spectroscopy1 (NMR) is limited by the signal-to-noise ratio, the measurement time Tm and the linewidth Δν = 1/(πT2). Overcoming the T2 limit is possible if the nuclear spins of a molecule emit continuous radio waves. Lasers2,3 and masers4,5,6,7,8,9,10,11,12,13 are self-organized systems which emit coherent radiation in the optical and micro-wave regime. Both are based on creating a population inversion of specific energy states. Here we show continuous oscillations of proton spins of organic molecules in the radiofrequency regime (raser5). We achieve this by coupling a population inversion created through signal amplification by reversible exchange (SABRE)14,15,16 to a high-quality-factor resonator. For the case of 15N labelled molecules, we observe multi-mode raser activity, which reports different spin quantum states. The corresponding 1H-15N J-coupled NMR spectra exhibit unprecedented sub-millihertz resolution and can be explained assuming two-spin ordered quantum states. Our findings demonstrate a substantial improvement in the frequency resolution of NMR.