Fig. 2: PDMR detection at ESLAC.

a Schematic describing the photoelectric readout principle at ESLAC. Only transitions responsible for the PDMR contrast between the different electron and nuclear spin states are visualized. The |m_s〉 = |+1〉 state, which was not probed in these experiments, is omitted for clarity. Under the application of the magnetic field (~510 G for ESLAC), the NV⁻ centre ground state (GS) energy levels |m_s〉 = |−1〉 and |m_s〉 = |0〉 are well separated (~1.5 GHz), whereas the excited state (ES) becomes nearly degenerate resulting in spin mixing between the states with the equivalent total spin projection quantum number. The spin mixing combined with the electron spin polarisation to |m_s〉 = |0〉 through the metastable state (MS) [grey arrows], results in the spin polarisation to the |m_s〉 = |0〉 electron and |m_I〉 = |+1〉 nuclear spin state. The yellow arrows depict optical transitions induced by the application of the yellow-green laser. As can be seen, the |m_s〉 = |0〉 spin sublevels in the ES are more likely to be excited by the second photon and contribute to the photocurrent by promoting the NV electron to the diamond conduction band (CB). When this happens, the negatively charged NV⁻ centre is converted to NV⁰ centre (red arrows). The back-conversion is possible by another two-photon process while preserving the nuclear spin orientation. First, the NV⁰ centre is excited to the ES and subsequently, an electron is promoted from the valence band (VB) to the vacated orbital of NV⁰, leading to the formation of NV⁻ centre. In this process the NV⁻ |m_s〉 = |0〉 ground states efficiently repolarise. b Pulsed PDMR measurements of the NV nuclear (¹⁴N) and electron (m_s = −1) spin hyperfine interaction for different magnetic fields showing nuclear spin polarisation close to the ESLAC (experimental conditions for measurement at 123 G—1500 ns laser pulse of 4 mW power, 1100 ns long MW π-pulse; for measurement at 439 G—1000 ns laser pulse of 6 mW power, 400 ns long MW π-pulse).