Quadrupolar\nResonance Spectroscopy of Individual Nuclei\nUsing a Room-Temperature Quantum Sensor

Nuclear quadrupolar resonance (NQR) spectroscopy reveals\nchemical\nbonding patterns in materials and molecules through the unique coupling\nbetween nuclear spins and local fields. However, traditional NQR techniques\nrequire macroscopic ensembles of nuclei to yield a detectable signal,\nwhich obscures molecule-to-molecule variations. Solid-state spin qubits,\nsuch as the nitrogen-vacancy (NV) center in diamond, facilitate the\ndetection and control of individual nuclei through their local magnetic\ncouplings. Here, we use NV centers to perform NQR spectroscopy on\ntheir associated nitrogen-14 (<sup>14</sup>N) nuclei at room temperature.\nIn mapping the nuclear quadrupolar Hamiltonian, we resolve minute\nvariations between individual nuclei. The measurements reveal correlations\nbetween the Hamiltonian parameters associated with the NV center’s\nelectronic and nuclear spin states, as well as a previously unreported\nsymmetry-breaking quadrupolar term. We further design pulse sequences\nto initialize, read out, and control the quantum evolution of the <sup>14</sup>N nuclear state using the nuclear quadrupolar Hamiltonian.