Abstract
Zero-field nuclear magnetic resonance (NMR) offers magnet-free access to nuclear spin-spin (scalar J) couplings, which define an intrinsic, molecule-specific frequency scale. However, the transient nature of zero-field NMR signals constrain spectral resolution and frequency stability. Here we introduce quantum J-oscillators that exploit J-couplings in molecules to produce phase-coherent continuous oscillations. Operated in zero magnetic field and driven by digital feedback, they generate sub-hertz to a few tens of hertz frequencies. In a proof-of-principle experiment on [15N]-acetonitrile, the oscillator achieves a 340 μHz linewidth over 3600 s, more than two orders of magnitude narrower than in conventional zero-field NMR. This methodology may facilitate precision measurements of J-coupling constants and enables discrimination of molecules whose zero-field NMR spectra are otherwise difficult to resolve. In addition, the combination of strongly coupled spin systems and programmable feedback turns J-oscillators into a compact tabletop platform for exploring nonlinear spin dynamics, including chaos and dynamical phase transitions. By uniting high-resolution spectroscopy and controllable quantum dynamics in a single, magnet-free setup, J-oscillators open new opportunities for applications where ultraprecise frequency references or molecular fingerprints are required.