There's a famous experiment displaying Heisenberg's Uncertainty Principle
Where a beam of light is subject to increasingly narrow slit (constraining its position), eventually causing it's momentum to be increasingly unclear (to maintain the inequality) spreading it out.
If I wanted to do the inverse of this experiment, where I have particles, I constrain the particles' momentum (somehow), and see that their positions are increasingly undefined, how would I do that?
If have a light beam originating from a source A to a target strip B,
the strip B is made increasingly small, so that if you spot a photon that landed on B, you would know the momentum to an increasing degree, thus the location of the photon at the source A should be increasingly unclear.
Now the question is how to measure this?