Fig. 3: Comparison of contrast and linewidth in MW cavity readout magnetic resonance and ODMR.

The signal associated with the NV⁻ \(\left|{m}_{s}=0\right\rangle \leftrightarrow \left|{m}_{s}=+1\right\rangle\) magnetic resonance is recorded simultaneously using MW cavity readout (blue solid line) and conventional optical readout (red solid line). The MW cavity readout realizes contrast C = 0.97, limited by imperfect circulator isolation, while conventional optical readout realizes contrast C = 0.05 (see Methods). For ease of comparison with the ODMR lineshape, MW cavity readout is performed here using a phase-insensitive measurement of reflected MW power, rather than the phase-sensitive technique; see Methods. Fits from the inhomogeneously-broadened numerical model (blue dashed line) and a Lorentzian model of ODMR (red dashed line) are also shown; see Supplementary Note 5. All ¹⁴N hyperfine transitions are included in both models, but the hyperfine structure is not resolved due to the substantial inhomogeneous broadening. The inset shows both readout signals scaled to the same peak-to-peak values, highlighting the ≈2 × narrowing of the magnetic resonance feature observed with MW cavity readout. The left-right asymmetry in the MW cavity readout signal is attributed to ≈−20 kHz detuning of the applied microwaves from the bare cavity resonance. The applied MW power is 10 dBm.