  We describe a procedure for engineering beams of cold atoms by selectively
draining particles from a trapped gas based on momenta. Atoms escape through a
filter potential that only transmits atoms with the desired momenta. We outline
an algorithm that outputs a filter potential that produces a pre-specified beam
momentum profile. We illustrate this procedure for the case of a narrow
band-pass (NBP) quantum filter. Lastly, we discuss the application of the NBP
filter for probing the self-energy and effective mass of Bose polarons, as well
as the corresponding Landau criterion.
