Fig. 2: Phase evolution of the target spin during the measurement.
From: High-resolution nanoscale NMR for arbitrary magnetic fields
The heterodyne detection schemes compares the phase of the Larmor precession of the target spin (blue line) with the phase of the coherent RF driving source (red line). During the free precession time τfid the nuclear spin acquires a phase of Δωτfid, where Δω is the detuning of the RF frequency from the Larmor frequency. The phase information (incorporated in 〈Ix〉, dark-green line) is then transferred to the Iz (green line) and read out via the sensor spin (yellow line) during the time τsens, before being transferred back to Ix. The application of a coherent RF source enables the effective down-conversion of the nuclear Larmor frequency from ωL to Δω. The evolution of the nuclear spin (red arrow) during one representative iteration of the measurement sequence, consisting of free evolution, first RF pulse, sensing period and second RF pulse, is also visualized in the bottom panel.
