Fig. 1: Concept of field independent heterodyne sensing by coherent sequential double resonance.
From: High-resolution nanoscale NMR for arbitrary magnetic fields
a The Nitrogen vacancy (NV) center with its internal 13C is studied as a model for a nano-scale surface Nuclear Magnetic Resonance (NMR) experiment with a diamond sensor. The target nuclei is coupled to the sensor via the longitudinal and transversal hyperfine coupling matrix elements Azz and Azx. b In the qdyne measurement schemes the nuclear precession is consequently probed by the electron spin during the measurement periods Mn. In our electron-nuclear-double-resonance quantum heterodyne (ENDOR qdyne) protocol the transverse polarization Ix of the nuclear spin is transferred to the z-axis with a \((\frac{\pi }{2})\) radio frequency (RF) pulse. The nuclear polarization along the z-axis can then be sensed by preparing the sensor electron spin in the superposition state with a \({(\frac{\pi }{2})}_{x}\) pulse and letting it evolve under the effective Hamiltonian \({\hat{S}}_{z}\otimes {\hat{I}}_{z}\), before a second pulse is applied and the sensor is read out. Afterwards the nuclear polarization along the z-axis is transferred back to Ix via a second RF pulse. Taking the polarization transfer of the nuclear spin due to the RF pulses into account, the effective Hamiltonian of the measurement is given by \({\hat{S}}_{z}\otimes {\hat{I}}_{x}\). In conventional qdyne schemes the (oscillating) transverse polarization of the nuclear spin is sensed directly via microwave (MW) pulse intensive dynamical decoupling (DD) sequences. In our experiment the Knill Dynamical Decoupling (KDD) sequence is used.
