Detection of J-spectra at zero field In Figures ) and 2(c), bubbling was performed at a sub-µT fields, polarizing the [ 15 N]-ACN molecules into the 15 N nuclear spin magnetization, Ŝx , Ŝy , or Ŝz , where the specific polarized state depends on the direction of the applied magnetic field during polarization (the so-called "SHEATH" polarization protocol. Depending on further magnetic field manipulations, we distinguish two different measurement sequences. In the "SHEATHadiabatic-ZF" sequence, polarization is read out by adiabatically reducing the applied field to zero and subsequently applying a DC pulse (see the Experimental Section for a calculation of the adiabatic field profile). Adiabatic field variation converts the initially prepared magnetization of N spins into the isotropic state Ŝ • K; application of the DC pulse allows reading out the signal, see Fig. ). The corresponding single-shot spectrum (SNR of 2J peak is ≈ 500, with noise level calculated using the 5-10 Hz range) is presented by the orange line in Fig. ). Alternatively, in the "SHEATH-sudden-ZF" sequence, Fig. (c), the polarization transfer field is nonadiabatically turned off (in around 5 µs), allowing nitrogen magnetization, Ŝz , to evolve under J-coupling at zero field. This gives rise to a similar spectrum albeit with different signal intensities of J and 2J peaks (see discussion below). The corresponding single-shot spectrum is presented with the blue line in Fig. . It is evident from the data in Fig. (d) that the SHEATH-pumping sequences provide a stronger signal compared to the ZF-pumping sequence. We attribute this phenomenon to a longer lifetime of Ŝz at ULF compared to the lifetime of Ŝ • K at ZF (Fig. ).