Physicists Collide Ultracold Atoms, Observe Pauli Exclusion

Physicists from New Zealand’s University of Otago have used steerable ‘optical tweezers’ to split minute clouds of ultracold atoms and slowly smash them together to directly observe the Pauli Exclusion, a key theoretical principle of quantum mechanics.

The principle places fundamental constraints on the behavior of groups of identical particles and underpins the structure and stability of atoms as well as the mechanical, electrical, magnetic and chemical properties of almost all materials.

Otago Physics researcher Associate Professor Niels Kjærgaard led the research, published in journal Nature Communications. His team cooled atomic clouds of fermionic potassium to temperatures a mere millionth of a Kelvin above absolute zero (T=1.3±0.1 μK) and then used precisely controlled laser beams to confine, accelerate and gently collide them.

The experiments unveiled a forbidden zone along a meridian of the spherical halo of scattered particles predicted by the Pauli Exclusion Principle. “This dark band results from a ‘no side-stepping’ rule that the principle dictates, which is that indistinguishable fermions cannot scatter out at 90 degrees to the collision axis,” says Kjærgaard.

When examining the data, however, PhD student Ryan Thomas found that this dark band would be less dark under some conditions, when the images of scattering halos from the particles actually displayed side-stepping instead.

“This is not because the rule suddenly breaks down, but because there can be situations where a particle scatters multiple times with consecutively new collision axes,” says Kjærgaard.

The team’s findings have important implications for research looking to gain insights into the particulars of the underlying processes governing multiple particle scattering.