  Single-molecule sensitivity of nuclear magnetic resonance (NMR) and angstrom
resolution of magnetic resonance imaging (MRI) are the highest challenges in
magnetic microscopy. Recent development in dynamical-decoupling- (DD) enhanced
diamond quantum sensing has enabled single-nucleus NMR and nanoscale NMR.
Similar to conventional NMR and MRI, current DD-based quantum sensing utilizes
the frequency fingerprints of target nuclear spins. The frequency fingerprints
by their nature cannot resolve different nuclear spins that have the same noise
frequency or differentiate different types of correlations in nuclear-spin
clusters, which limit the resolution of single-molecule MRI. Here we show that
this limitation can be overcome by using wave-function fingerprints of target
nuclear spins, which is much more sensitive than the frequency fingerprints to
the weak hyperfine interaction between the targets and a sensor under resonant
DD control. We demonstrate a scheme of angstrom-resolution MRI that is capable
of counting and individually localizing single nuclear spins of the same
frequency and characterizing the correlations in nuclear-spin clusters. A
nitrogen-vacancy-center spin sensor near a diamond surface, provided that the
coherence time is improved by surface engineering in the near future, may be
employed to determine with angstrom resolution the positions and conformation
of single molecules that are isotope labeled. The scheme in this work offers an
approach to breaking the resolution limit set by the frequency gradients in
conventional MRI and to reaching the angstrom-scale resolution.
