# How do you experimentally constrain momentum?

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?

One idea:

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?

Fun fact: the formula you learned in the intro class $$r_c = \frac{p}{qB} \;,$$ remains correct for relativistic particles provided you use the momentum in the numerator rather than $mv$ (which—sadly—is the way many intro texts express it; tsk tsk).