The NMR samples are thermally polarized for 20 s using a 1.8-T magnet placed above the magnetic shield, and mechanically shuttled into a zero-field detection region (inside the magnetic shield), where the magnetic field of the nuclear spins is measured using a home-built alkalivapor atomic magnetometer. During the transfer, taking roughly 300 ms (plus an additional 700-ms delay), a guiding field of roughly 10 µT is applied by a solenoid coil wrapping the whole length of the shuttling path. When the sample reaches the detection area, the guiding field is turned off suddenly to generate an oscillating NMR signal . Each zero-field NMR spectra is the result of averaging 2048 transients. The entire data processing is performed using Python. A comprehensive description of the experimental setup and a detailed explanation of data processing can be found in the previous work .