  The precise measurement of mechanical stress at the nanoscale is of
fundamental and technological importance. In principle, all six independent
variables of the stress tensor, which describe the direction and magnitude of
compression/tension and shear stress in a solid, can be exploited to tune or
enhance the properties of materials and devices. However, existing techniques
to probe the local stress are generally incapable of measuring the entire
stress tensor. Here, we make use of an ensemble of atomic-sized in-situ strain
sensors in diamond (nitrogen-vacancy defects) to achieve spatial mapping of the
full stress tensor, with a sub-micrometer spatial resolution and a sensitivity
of the order of 1 MPa (corresponding to a strain of less than $10^{-6}$). To
illustrate the effectiveness and versatility of the technique, we apply it to a
broad range of experimental situations, including mapping the elastic stress
induced by localized implantation damage, nano-indents and scratches. In
addition, we observe surprisingly large stress contributions from functional
electronic devices fabricated on the diamond, and also demonstrate sensitivity
to deformations of materials in contact with the diamond. Our technique could
enable in-situ measurements of the mechanical response of diamond
nanostructures under various stimuli, with potential applications in strain
engineering for diamond-based quantum technologies and in nanomechanical
sensing for on-chip mass spectroscopy.
