Published online by Cambridge University Press: 08 November 2010
A defining feature of quantum mechanics is that it allows systems to exist in a superposition of different eigenstates of certain observables such as position or spin. However, superpositions of other quantities such as mass or charge are not seen in nature. It is thought that this disparity is partly due to the fact that it is much easier to carry out interference experiments for certain observables than others. Here we present an interferometry scheme that should allow us to observe interference between the vacuum and a single photon or atom. We begin by presenting a scheme for a Hadamard gate that operates in the Fock state basis and then show how, by creating an interferometer from two such gates, interference between a single particle and the vacuum could indeed be observed. This would provide evidence of a superposition of different particle numbers.