Fig. 1. Theoretical framework overview.

Polarization of a bath of nuclear spins (gray) by an electron spin (yellow): The initially unpolarized bath and the polarized electron spin (upper left) evolve according to an engineered Hamiltonian that describes a flip-flop interaction, and the electron spin is periodically reinitialized. After several repetitions, the nuclear spin bath is polarized (lower right). Illustration of effective Hamiltonian engineering (gray box): A combination of standard symmetric and asymmetric DD sequences with the effective Hamiltonians H ∝ S_z ⊗ I_x/y can be modified to a flip-flop Hamiltonian H ∝ S_x ⊗ I_x + S_y ⊗ I_y by introducing basis changes with π/2 pulses.