(color online) Protection of center spin coherence by dynamic nuclear spin polarization (DNP). (a) Illustration of the dynamically polarized nuclear spin bath (purple) around an NV center (red). The spin polarization is first built by the green laser pulse, then it can be transfer to the surrounding ¹³C nuclear spins under certain conditions. (b) At excited-state level anti-crossing (ESLAC) point, the energy mismatch between the electron spin and nearby nuclear spins is compensated by the external magnetic field (about 500 Gs). (c) The Hartman–Hahn condition. (d) Experimental measured FID signals without (black line) and after (red line) DNP, the prolongation of the dephasing time (from to ) indicates that the bath spin is polarized during the DNP process. (e) Dependence of the time on the DNP pumping duration. Reproduced from Ref. [21], RSC.