  Sterile neutrinos are natural extensions to the standard model of particle
physics and provide a possible portal to the dark sector. We report a new
search for the existence of sub-MeV sterile neutrinos using the decay-momentum
reconstruction technique in the decay of $^7$Be. The experiment measures the
total energy of the $^7$Li daughter atom from the electron capture decay of
$^7$Be implanted into sensitive superconducting tunnel junction (STJ) quantum
sensors. This first experiment presents data from a single STJ operated at a
low count rate for a net total of 28 days, and provides exclusion limits on
sterile neutrinos in the mass range from 100 to 850 keV that improve upon
previous work by up to an order of magnitude.
