Figure 3. Sensing a single nuclear spin.

(a,b) Colour-coded memory qubit signals (that is, probability of detecting state ) of enhanced correlation measurement with an RF pulse during T_c=1 ms. Sensing time and RF pulse frequency offset Δν to the bare ¹³C Larmor frequency are varied (cf. Mims electron-nuclear-double-resonance (ENDOR)²⁹). A resonance of a ¹³C spin around Δν=89 kHz appears, where, for increasing , the phase Δφ of the memory increases. The width of the ¹³C spin controlled phase accumulation of about 1 kHz is governed by the RF pulse duration. a,b show cos Δφ and sin Δφ of the memory phase because the phase of the final memory qubit π/2-pulse is set to 0 (a) and π/2 (b) (see Fig. 1c). Please note, that for the measurements done with (b) initialization of the ¹³C spin is required (here: spin down, see main text). Variations of the background in panel (b) are due to drifts of Δω (see equation 6, see the ‘Methods' section). (c,d) On resonance, the RF pulse amplitude is varied, to induce Rabi oscillations of the detected ¹³C spin. Phases are 0 in (c) and ±π/2 in (d) for circles and squares.