  We demonstrate flux-bias locking and operation of a gradiometric fluxonium
artificial atom using two symmetric granular aluminum (grAl) loops to implement
the superinductor. The gradiometric fluxonium shows two orders of magnitude
suppression of sensitivity to homogeneous magnetic fields, which can be an
asset for hybrid quantum systems requiring strong magnetic field biasing. By
cooling down the device in an external magnetic field while crossing the
metal-to-superconductor transition, the gradiometric fluxonium can be locked
either at $0$ or $\Phi_0/2$ effective flux bias, corresponding to an even or
odd number of trapped fluxons, respectively. At mK temperatures, the fluxon
parity prepared during initialization survives to magnetic field bias exceeding
$100 \,\Phi_0$. However, even for states biased in the vicinity of $1
\,\Phi_0$, we observe unexpectedly short fluxon lifetimes of a few hours, which
cannot be explained by thermal or quantum phase slips. When operating in a
deep-underground cryostat of the Gran Sasso laboratory, the fluxon lifetimes
increase to days, indicating that ionizing events activate phase slips in the
grAl superinductor.
