Nuclear magnetic resonance (NMR) imaging and spectroscopy are powerful tools for investigating millimeter-to-meter-sized objects with three-dimensional spatial resolution and chemical specificity. Over the last few years, novel detection techniques have paved the way for extending NMR to the nanometer scale, affording tremendous opportunities in structural biology and analytical chemistry. A particularly fruitful approach relies on NMR detection using single defect spins in diamond. This approach adopts quantum techniques known as multipulse decoupling sequences to detect different NMR isotopes based on spectral identification. We report a subtle yet critical feature of the multipulse ac sequences: the uniqueness of the spectral assignment.

We find that the commonly used detection sequences generate signals not only at the fundamental frequency but also at many harmonics and subharmonics. These additional features make it difficult and sometimes impossible to uniquely identify NMR signals. We use numerical simulations to determine how the system’s Hamiltonian evolves, and we also experimentally test our theory using a nitrogen-vacancy center as a single spin sensor. The harmonics result in ambiguity when we try to identify a heterogeneous group of atoms, particularly for certain isotopes of hydrogen, carbon, phosphorus, and silicon.

Our work has implications on past and future work in diamond-based NMR and will be of critical interest to the field of “quantum sensing” in general.