  Nitrogen-vacancy (NV) quantum magnetometers offer exceptional sensitivity and
long-term stability. However, their use to date in distributed sensing
applications, including remote detection of ferrous metals, geophysics, and
biosensing, has been limited due to the need to combine optical, RF, and
magnetic excitations into a single system. Existing approaches have yielded
localized devices but not distributed capabilities. In this study, we report on
a continuous system-in-a-fiber architecture that enables distributed magnetic
sensing over extended lengths. Key to this realization is a thermally drawn
fiber that has hundreds of embedded photodiodes connected in parallel and a
hollow optical waveguide that contains a fluid with NV diamonds. This fiber is
placed in a larger coaxial cable to deliver the required RF excitation. We
realize this distributed quantum sensor in a water-immersible 90-meter-long
cable with 102 detection sites. A sensitivity of 63 nT Hz-1/2 per site, limited
by laser shot noise, was established along a 90 m test section. This fiber
architecture opens new possibilities as a robust and scalable platform for
distributed quantum sensing technologies.
