Fig. 4: Nitrogen nuclear spin control and NV–P1 coupling sign.

a ¹⁴N Rabi oscillation. Top: energy levels of the P1 electron spin in the {0D, +1D} subspace. Bottom: either S1 or S2 is prepared in \(\left|+1,{\rm{D}}\right\rangle\) and the length t of a pulse at frequency RF = RF_14N = 36.8 MHz is varied. The nitrogen nuclear spin is driven conditionally on the electron spin state. Inset: NMR spectrum obtained by varying the frequency RF for a fixed pulse duration t. b We use the ¹⁴N spin to determine the sign of the NV–P1 coupling. First, we prepare a selected P1 center (K=820) and initialize its electron spin (L=2). Second, we apply a π pulse at RF_14N, which implements an electron-controlled CNOT_e,N (see level structure in a). The coupling sign to the NV determines the P1 electron–spin state, and, in turn, the final ¹⁴N state. Finally, we measure the fidelity with the ¹⁴N \(\left|+1\right\rangle\) state for two opposite electron spin initializations (+y and −y final π/2 pulse of DEER(y)). The normalized difference R of these measurements reveals the sign of the coupling (see “Methods”). All error bars indicate one statistical standard deviation.