Magnetic imaging with nitrogen-vacancy (NV) spins in
diamond is
becoming an established tool for studying nanoscale physics in condensed
matter systems. However, the optical access required for NV spin readout
remains an important hurdle for operation in challenging environments
such as millikelvin cryostats or biological systems. Here, we demonstrate
a scanning-NV sensor consisting of a diamond nanobeam that is optically
coupled to a tapered optical fiber. This nanobeam sensor combines
a natural scanning-probe geometry with high-efficiency through-fiber
optical excitation and readout of the NV spins. We demonstrate through-fiber
optically interrogated electron spin resonance and proof-of-principle
magnetometry operation by imaging spin waves in an yttrium-iron-garnet
thin film. Our scanning-nanobeam sensor can be combined with nanophotonic
structuring to control the light–matter interaction strength
and has potential for applications that benefit from all-fiber sensor
access, such as millikelvin systems.