Being really close ...

Interference is one of the basic phenomena in physics. It describes the ability of waves to superimpose constructively or destructively. How the waves interfere depends on the phase relation between the waves. Interference is observed for water waves, sound waves and light but also for matter waves. The latter is described by the fundamental laws of quantum mechanics.

A well-known interference experiment is light diffraction on a periodic grating. Such an experiment can be performed very easily at home: If the light of a laser pointer shines onto a DVD or CD the reflected light will show a characteristic, periodic spot pattern on screen. As the screen is usually far away from the CD one describes this phenomenon as far-field interference.

However, the question arises how the diffraction pattern looks like in the vicinity of the CD – in the near-field. This question was solved first by William Henry Fox Talbot in 1836, when he showed that in well-defined distances from the grating an exact image is produced. The distance is thereby modulated by the lattice constant of the grating and the wavelength of the light. In the previous example a few micrometers above the CD surface one finds a sharp image of the CD itself. This is the so-called Talbot effect.

An important difference between interference in the far- and near-field is that for the far-field all partial waves interfere with each other, while in the near-field only adjacent waves interfere. Therefore, near-field or Talbot interferometry allows to study the phase relation between adjacent waves.

This effect has been used to study the time-dependent phase evolution of matter waves by a cooperation within the State Research Center OPTIMAS between Professor Herwig Ott and outside lecturer Axel Pelster from the theoretical group of Professor Sebastian Eggert. By using a redesigned version of the Talbot interferometry they could show for the first time how a large scale matter wave evolves from statistical noise slightly above the absolute zero in temperature in interacting gas of rubidium atoms. This experiment by the researchers in Kaiserslautern is a big leap towards a better understanding of complex quantum dynamics of interacting many-body systems. Furthermore, they developed a whole new experimental method to address the correlation between quantum mechanical phases, which can be now used by a large number of other groups worldwide.

The work was carried out within the Collaborative Research Center TR49 “Condensed Matter Systems with Variable Many-Body Interactions” as well as OSCAR “Open System Control of Atomic and Photonic Matter” and has been published in “Nature Communications”:

TITLE: Measuring Finite-Range Phase Coherence in an Optical Lattice Using Talbot Interferometry
https://www.nature.com/articles/ncomms15601

Contact person:

Prof. Dr. Herwig Ott
E-mail: ott[at]physik.uni-kl.de
Phone: 0631 205 2817