Information obtained from quantum noise
Using a correlation analysis, physicists working with Immanuel Bloch in Mainz have succeeded in examining the quantum noise in images of atomic gas clouds and in gaining information about the original arrangement of the atoms.
The scientists had cooled thin clouds of potassium atoms to temperatures just above absolute zero and trapped them in an optical lattice created by superimposed laser beams. As fermions, neutral potassium atoms adhere to the Pauli principle, according to which identical particles cannot occupy the same quantum state in groups – they are therefore strict loners. In the "crystal of light" they arrange themselves along the crystal axes and thus form an ordered structure.
When the laser light is switched off, the atoms can move freely again and the structure disintegrates. After a while they have moved so far apart that a photograph of the cloud now shows the quantum noise. By determining correlations between individual atoms in the noise image, the researchers were able to draw conclusions about the previous order.
In 1956, Robert Hanbury Brown and Richard Twiss demonstrated correlations between photons and observed for the first time the "bunching" typical of bosons - the preferred occurrence together. The noise correlations allowed the scientists to derive information about the properties of the light source. Bloch and his co-workers have now succeeded in observing the "antibunching" of fermions. Because of the original arrangement in the optical lattice, there is no other fermion at certain distances from it. The method should therefore be suitable for detecting more complex atomic orders. Perhaps this will help clarify questions about high-temperature superconductivity, for example. (af)