Field variation steps can be introduced when different pumping and detection fields are applied. If the field changes over a time scale such that the spin order remains unchanged, this is referred to as a "sudden" field variation. In contrast, when the field varies slowly enough for the polarization to evolve adiabatically as the system undergoes evolution through level anticrossings, it is called an "adiabatic" field variation. Furthermore, as detailed subsequently, a simple field-switching sequence can be employed between the SHEATH-pumping and ULF detection. In this scenario termed ZF mixing, the system is brought to zero field between the pumping and detection phases, allowing polarization to transfer spontaneously between different nuclear species. , pH 2 bubbling through the sample directly at zero field) or SHEATHpumping (i.e., pH 2 bubbling at a variable elevated magnetic field), field variation steps, and signal detection with an atomic magnetometer initiated with a d.c. pulse optimized to amplify the spin order in question; (c) Map of various spin orders (laboratory frame product operators) generated during preparation, field variation, and signal detection stages of the experiment. The initial state is highlighted in orange. Intermediate spin orders formed on N and 1 H nuclei are denoted in blue and green, respectively. The final readout spin order is indicated in purple. different experimental stages. Arrows denote transfer directions, with numbers indicating the stage of experiment (Fig. ). The flow of spin orders is analyzed using the laboratoryframe product operator formalism (distinct from rotating-frame conventions in high-field NMR), where Ŝ and K represent the 15 N and total 1 H spin operators of acetonitrile, respectively. The initial state (scalar order of two hydrogen atoms, orange) first evolves to intermediate spin orders on N nucleus (blue), 1 H nuclei (green), and their correlated product operators. These intermediate states are converted to the final observable spin orders (purple). Details of the spin-order transfer/control are explained in the Results Section.