  Some hypothetical particles are considered essentially undetectable because
they are far too light and slow-moving to transfer appreciable energy or
momentum to the normal matter that composes a detector. I propose instead
directly detecting such feeble particles, like sub-MeV dark matter or even
gravitons, through their uniquely distinguishable decoherent effects on quantum
devices like matter interferometers. More generally, decoherence can reveal
phenomena that have arbitrarily little classical influence on normal matter,
giving new motivation for the pursuit of macroscopic superpositions.
