  Crystal strain variation imposes significant limitations on many quantum
sensing and information applications for solid-state defect qubits in diamond.
Thus, precision measurement and control of diamond crystal strain is a key
challenge. Here, we report diamond strain measurements with a unique set of
capabilities, including micron-scale spatial resolution, millimeter-scale
field-of-view, and a two order-of-magnitude improvement in volume-normalized
sensitivity over previous work [1], reaching $5(2) \times
10^{-8}/\sqrt{\rm{Hz}\cdot\rm{\mu m}^3}$ (with spin-strain coupling
coefficients representing the dominant systematic uncertainty). We use
strain-sensitive spin-state interferometry on ensembles of nitrogen vacancy
(NV) color centers in single-crystal CVD bulk diamond with low strain
gradients. This quantum interferometry technique provides insensitivity to
magnetic-field inhomogeneity from the electronic and nuclear spin bath, thereby
enabling long NV ensemble electronic spin dephasing times and enhanced strain
sensitivity. We demonstrate the strain-sensitive measurement protocol first on
a scanning confocal laser microscope, providing quantitative measurement of
sensitivity as well as three-dimensional strain mapping; and second on a
wide-field imaging quantum diamond microscope (QDM). Our strain microscopy
technique enables fast, sensitive characterization for diamond material
engineering and nanofabrication; as well as diamond-based sensing of strains
applied externally, as in diamond anvil cells or embedded diamond stress
sensors, or internally, as by crystal damage due to particle-induced nuclear
recoils.
