[mnl]

This special molecule of Stephen's is essentially a Maxwell's demon (it could use that information to open or close the door by choosing appropriate collisions, or simply act as the gate itself). There's a lot of literature about that.

Actually Stephen's special molecule is more powerful because it's omniscient. Ordinary Maxwell's demons just see fast or slow molecules coming at the gate and act accordingly. This one knows the momentum and position of every other particle it needs to know something about, which can be peculiar if you don't think about uncertainty.