Fig. 4: Low-power microwave sensing.
From: Bipartite entanglement in a nuclear spin register mediated by a quasi-free electron spin
a Continuously decoupled low-power Rabi driving showing persistent oscillations at a frequency of ΩR,e/2π = 70.93(59) kHz beyond the dephasing time \({T}_{2,{{\mathrm{e}}}}^{*}=5.69(40)\,{\mu {{\mathrm{s}}}}\). Solid line is a fit to \(a\sin ({\Omega }_{{{\mathrm{R}}},{{\mathrm{e}}}}{T}_{{\mathrm{MW}}}+\phi )\exp (-{T}_{{\mathrm{MW}}}/{T}_{2,{{\mathrm{e}}}}^{{\mathrm{Rabi}}})+c\) b Frequency scan of a π-pulse with ΩR,e from (a) reveals eight resonances which are fit with \(a\,{\sum }_{i=1}^{8}{{\mathcal{L}}}({\nu }_{MW},{\nu }_{0}^{i},\Gamma )+c\) (solid line), where \({{\mathcal{L}}}({\nu }_{MW},{\nu }_{0}^{i},\Gamma )\) is a Lorentzian. c Polarizing the three nuclear spins from (b) before scanning the frequency at ΩR,e/2π = 7.7 kHz increases sensitivity below the unpolarized inhomogeneous linewidth and shows a fourth nuclear spin with \({A}_{\parallel }^{4}/2\pi=32.5(15)\,{{\mathrm{kHz}}}\), extracted from a double-Lorentzian (solid line).
