When an electron moves through a molecule, we can only predict its behaviour by assuming that it follows a quantum superposition of paths. However, on an everyday scale superpositions are never encountered. What happens to moving objects at an intermediate scale of millions of atoms?

We have proposed a nanomechanical on-chip interferometer in which the particle under test is and entire nanotube, containing thousands to millions of atoms (see figure). It uses a suspended nanotube as one SQUID arm of a transmon-type qubit (a ‘gatemon’). We are now planning to implement this proposal. This would be a landmark demonstration of quantum behaviour, demonstrating that spatial quantum interference applies to masses a thousand times larger than in the best molecular-beam interferometers.

Figure: The “displacemon” proposal for measuring mechanical quantum interference. (a) Circuit diagram. (b) Device geometry. Modulation of the magnetic flux by the nanotube’s motion gives rise to strong qubit-mechanical coupling.