In a second step, to study the coherent nature of PHIP transfer at ULF, we conduct experiments according to protocol 2 (Fig. .c): (1) pH2 bubbling at field 𝐵 𝑗𝑢𝑚𝑝 to accumulate PHIP on protons, (2) instant field jump to ultralow field 𝐵 𝑈𝐿𝐹 with subsequent free evolution for time 𝜏 𝑈𝐿𝐹 , (3) instant field jump to a field 𝐵 𝑗𝑢𝑚𝑝 , (4) sample transfer to the detection zone, and (5) C NMR detection with a hard 90 pulse. Polarization transfer efficiency in this type of experiment depends on two parameters, i.e., on the ULF field strength and the evolution period. The optimal 𝐵 𝑈𝐿𝐹 value here is the same as in protocol 1 and can be easily determined either experimentally or by numerical calculation. Hence, for this type of transfer, we only studied the dependence of the resulting C polarization on the evolution period.