One of the most basic but profound paradigms of quantum mechanics is superposition. It is an essential ingredient for quantum interference and can give rise to entanglement. In the prototypical experiment a particle (or an entire system) is cast into a superposition of internal states.

Here we show that the notion of superposition reaches much further by creating a coherent superposition of diatomic and triatomic molecules – chemically different bound states. Since the three-body problem is intrinsically non-separable, i.e. it cannot be written in the form of one effective particle, this goes beyond previous realizations of superposition.

More specifically, by means of rf-modulation we cast a system of three Li-7 atoms in an ultracold atomic gas into a superposition state where either two atoms form a Feshbach dimer and one is free or all three form an Efimov trimer. By measuring the coherent evolution in an interferometer-like experiment we demonstrate the coherence of the superposition state. As an application we are able to determine the binding energy of the Efimov trimer with unprecedented precision in a so-far inaccessible parameter regime.