An international team of scientists, led by Dr. Niels Kjærgaard from the University of Otago in Dunedin, New Zealand, has used steerable ‘optical tweezers’ to split ultracold clouds of potassium-40 (40K) atoms and smash them together to directly observe a key principle of quantum mechanics – the Pauli exclusion principle.
The Pauli exclusion principle predicts a forbidden zone along a meridian of the spherical halo of scattered particles, which the experiments indeed unveiled. The dark band in the graphic shows a rule derived from the principle in action. This rule is that indistinguishable fermions cannot scatter out at 90 degrees to the collision axis. Image credit: Niels Kjærgaard.
The Pauli exclusion principle was proposed in 1925 by Austrian physicist Wolfgang Pauli.
It states that, in an atom or molecule, no two electrons can have identical quantum numbers. As an orbital can contain a maximum of only two electrons, the two electrons must have opposing spins.
The principle underpins the structure and stability of atoms as well as the mechanical, electrical, magnetic and chemical properties of almost all materials.
Dr. Kjærgaard and his colleagues from the United States, Denmark and New Zealand used extremely precisely controlled laser beams to confine, accelerate and collide ultracold atomic clouds of fermionic 40K.
The atomic clouds had a temperature of a mere millionth of degree Kelvin above absolute zero.
The Pauli exclusion principle predicts a forbidden zone along a meridian of the spherical halo of scattered particles, which the team’s experiments indeed unveiled.
“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,” Dr. Kjærgaard said.
When Dr. Kjærgaard and co-authors looked more closely at the data, they found that under some conditions the images of scattering halos from the particles would actually display side-stepping — the dark band would be less dark.
“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,” Dr. Kjærgaard said.
“This particular finding has important implications for gaining insights into the particulars of the underlying processes governing multiple particle scattering.”
The scientists describe their results in the July 11 issue of the journal Nature Communications.
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R. Thomas et al. 2016. Multiple scattering dynamics of fermions at an isolated p-wave resonance. Nature Communications 7, article number: 12069; doi: 10.1038/ncomms12069
