Fig. 4: Pulse sequence for calibration of measurement direction.

The RF field pulse between the pump and probe laser pulses creates a transverse spin excitation, rotating in the y-z plane. The first part of probe pulse measures \({\hat{p}}_{\text{A}}\). By tuning the phase of the RF field to maximize the first probe signal, we can validate that the transverse spin is in the \({\hat{p}}_{\text{A}}\) direction (in the rotating frame). If the second probe signal, after a quarter of the Larmor period, is minimal, we verify that the induced spin is in the \({\hat{x}}_{\text{A}}\)-direction.