  Nuclear spin imaging at the atomic level is essential for the understanding
of fundamental biological phenomena and for applications such as drug
discovery. The advent of novel nano-scale sensors has given hope of achieving
the long-standing goal of single-protein, high spatial-resolution structure
determination in their natural environment and ambient conditions. In
particular, quantum sensors based on the spin-dependent photoluminescence of
Nitrogen Vacancy (NV) centers in diamond have recently been used to detect
nanoscale ensembles of external nuclear spins. While NV sensitivity is
approaching single-spin levels, extracting relevant information from a very
complex structure is a further challenge, since it requires not only the
ability to sense the magnetic field of an isolated nuclear spin, but also to
achieve atomic-scale spatial resolution. Here we propose a method that, by
exploiting the coupling of the NV center to an intrinsic quantum memory
associated with the Nitrogen nuclear spin, can reach a tenfold improvement in
spatial resolution, down to atomic scales. The spatial resolution enhancement
is achieved through coherent control of the sensor spin, which creates a
dynamic frequency filter selecting only a few nuclear spins at a time. We
propose and analyze a protocol that would allow not only sensing individual
spins in a complex biomolecule, but also unraveling couplings among them, thus
elucidating local characteristics of the molecule structure.
