In our group we use the coherence of the conduction electrons to investigate solid state properties. Conduction electron interference permits us, for example, to determine the time development of the electron spin orientation on a picosecond time scale. This method is particularly suited to detect and investigate magnetic impurities in metals. It is by a factor of 100 more sensitive than competing methods. We discovered this method accidentally in the late 70's when investigating the anomalous Hall effect of Pd films, covered with magnetic impurities /1/. In the 80's this phenomenon became rather famous under the name quantum interference or weak localization . In the following the basic physics is explained in simple steps. For a more detailed discussion see the review article /2/.
Assume that you have a city with streets like a chess board. In the center of the town is a bar which is crowded with drunken sailors. Now assume that all the sailors leave the bar at the time t=0. In Fig.1 we follow the path of such a sailor. At each corner he makes a random turn because he does not remember where he came from. There is a finite probability that the sailor returns (by accident) back to the original bar. As a matter of fact the bouncer (in front of the bar) sees quite a few sailors passing by. Although each sailor walks randomly through the city the number of sailors which return accidentally to the bar is inversely proportional to the passed time, i.e. proportional to 1/t.