Standing in front of a mirror, we can easily tell apart ourselves from our mirror image. The mirror does not affect our motion in any way. For quantum particles, this is much more complicated. In a spectacular experiment in the labs of the Heidelberg University, a group of physicists from Heidelberg Unversity, together with colleagues at TU Munich and TU Vienna extended a gedankenexperiment by Einstein and managed to blur the distinction between a particle and its mirror image. The results of this experiment have now been published in the journal “Nature Physics”.
When an atom emits light (i.e. a photon) into a particular direction, it recoils in the opposite direction. If the photon is measured, the motion of the atom is known too. The scientists placed atoms very closely to a mirror. In this case, there are two possible paths for any photon travelling to the observer: it could have been emitted directly into the direction of the observer, or it could have travelled into the opposite direction and then been reflected in the mirror. If there is no way of distinguishing between these two scenarios, the motion of the atom is not determined, the atom moves in a superposition of both paths.
http://www.uni-heidelberg.de/presse/news2011/pm20110404_quanten_spiegelbild_en.html
