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 University of Heidelberg, a group of physicists at the University Heidelberg, together with colleagues at TU Munich and TU Vienna extended a ‘thought experiment’ by Einstein and managed to blur the distinction between a particle and its mirror image.
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.
“If the distance between the atom and the mirror is very small, it is physically impossible to distinguish between these two paths,” Jiri Tomkovic, PhD student at Heidelberg explains. The particle and its mirror image cannot be clearly separated any more. The atom moves towards the mirror and away from the mirror at the same time. This may sound paradoxical and it is certainly impossible in classical phyiscs for macroscopic objects, but in quantum physics, such superpositions are a well-known phenomenon.
“This uncertainty about the state of the atom does not mean that the measurement lacks precision,” Jörg Schmiedmayer (TU Vienna) emphasizes. “It is a fundamental property of quantum physics: The particle is in both of the two possible states simultaneousely, it is in a superposition.” In the experiment the two motional states of the atom — one moving towards the mirror and the other moving away from the mirror — are then combined using Bragg diffraction from a grating made of laser light. Observing interference it can be directly shown that the atom has indeed been traveling both paths at once.