Single atom resolved fluorescence image of a band insulator of fermionic lithium in an optical lattice. Credit: MPQ, Quantum Many Body Systems Division
Today, quantum optical experiments provide methods to prove the rules math equations. In this regard, scientists in the Quantum Many-Body Division at Max Planck Institute of Quantum Optics have made a big step forward.
The “exclusion principle” was formulated by the Austrian physicist Wolfgang Pauli in 1925 in order to explain the structure and the stability of atoms. Dr. Christian Groß’s team demonstrated the first direct observation of “Pauli blocking”, a consequence of the exclusion principle. To this end, they cooled a cloud of fermionic lithium-6 atoms down to extremely low temperatures and loaded the particles into an optical lattice. Since identical fermions are not allowed to occupy the same lattice site each atom is supposed to find its own place. This was exactly what was observed in the experiment with the help of a quantum gas microscope that images atoms with single-particle single-site resolution. “Our success is the result of adapting our cooling and imaging methods that were developed for bosons, to the needs of fermions”, Christian Groß explains. “Our work opens a new avenue for studying quantum correlations in fermionic quantum matter, or to get a better understanding of phenomena such as quantum magnetism and superconductivity.”
Quantum statistics distinguishes between 2 fundamentally different kinds of particles. There are, “sociable” bosons which condense into a single quantum state at 0 temp. Then there’s “solitary” fermions for which multiple occupation of a single state is forbidden. In their work with cold quantum gases scientists have a free choice: particles with integer spins are bosons, whereas fermions are characterized by their half-integer spin. Hence, what kind of statistics, i.e. what ‘social behaviour’ the atoms obey depends on the total number of their electrons, protons, and neutrons. If the goal is to simulate the behaviour of electrons in a solid crystal with atoms in optical lattices, fermions are, of course, the better approximation. However, in most experiments so far scientists have used bosonic particles since it is, for several reasons much harder to bring fermions down to the low temperatures required.
At first, the fermionic lithium-6 atoms are cooled down and caught in a dipole trap. By applying several light-fields they obtain a single plane with a couple of hundred atoms. Now an optical lattice – created by interfering laser beams – is superimposed and defines the “crystal geometry”, ie sites where atoms are allowed to settle down.
Photons are scattered at the atoms, causing them to light up like tiny nano light bulbs that can each be observed individually. A high resolution quantum gas microscope objective images the atoms all at once. The measurements show a rather flat distribution in the centre of the trap, with one atom per lattice site. “It is important that this distribution arises as a result of quantum statistics – i.e. Pauli blocking – only,” Ahmed Omran points out, doctoral candidate at the experiment. “Identical fermions repel each other; there is no other interaction at work.”
Due to the periodic order of atoms in a solid state crystal the energy levels of the electrons split up into “bands” of closely neighboured states. If the highest valence band is fully occupied the electrons cannot move, ie insulator. The quantum state of the fermionic lithium-system shows the Pauli principle leads to a fully occupied valence band, i.e. to a suppression of conductivity, with a strong suppression of particle number fluctuations detected with the quantum gas microscope.
Modifications of the method will make it possible to manipulate even single fermions in the systems, leading to even lower temperatures. There a new antiferromagnetic order is expected to emerge that should be detected and characterized with the quantum gas microscope. Antiferromagnetism is a candidate for phenomena in superconductivity.
http://phys.org/news/2016-01-cold-fermions-distance.htmljCp
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