By placing an atom very close to a mirror, and having the atom emits a photon either away, or towards the mirror, physicists at Heidelberg University, TU Munich, and TU Vienna have shown a superposition of an atom that is moving simultaneously in two opposite directions.
“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.
I still have trouble understanding how the interference pattern can infer the superposition motion of the atom. To me, the photon itself will "self-interfere" since each one of them are in superposition of both paths (i.e. emitted away from the mirror and emitted towards the mirror and gets reflected back). Thus, when combined, they will self-interfere, very much like the double slit. So why the need for the Bragg diffraction grating?
Like I said, I definitely need to read the paper, since I am obviously missing something here.
Still, being able to show the atom having that opposite motion simultaneously is amazing. It is very much like the Delft/Stony Brook SQUID experiments, where the supercurrent was moving in two opposite directions at the same time.