  Inertial sensors that measure the acceleration of ultracold atoms promise
unrivalled accuracy compared to classical equivalents. However, atomic systems
are sensitive to various perturbations, including magnetic fields, which can
introduce measurement inaccuracies. To address this challenge, we have
designed, manufactured, and validated a magnetic field stabilisation system for
a quantum sensor based on atom interferometry. We solve for the magnetic field
generated by surface currents in-between a pair of rectangular coils and
approximate the surface current using discrete wires. The wires are wound
by-hand onto machined panels which are retrofitted onto the existing mounting
structure of the sensor without interfering with any experimental components.
Along the central $60$ mm of the $y$-axis, which aligns with the trajectory of
the atoms during interferometry, the coils are measured to generate an
independent uniform axial magnetic field with a strength of
$B_z=\left(22.81\pm0.01\right)$ $\mu$T/A [$\mathrm{mean}\pm2\sigma\mathrm{std.
error}$] and an independent linear axial field gradient of strength
$\mathrm{d}B_z/\mathrm{d}y=\left(10.6\pm0.1\right)$ $\mu$T/Am. The uniform
$B_z$ field is measured to deviate by a maximum value of $1.3$% in the same
region, which is a factor of three times more uniform than the previously-used
on-sensor rectangular $B_z$ compensation set.
