Observation of the Quantum Gouy Phase

Photonic N00N states, i.e., states of light where N photons are in an extremal superposition between two orthogonal states $\frac{1}{\sqrt{2}}(\vert N,0\rangle +\vert 0,N\rangle)$ , have an increased phase-sensitivity in comparison to their classical counterparts. Using the sensitivity offered by N00N states, in conjunction with the intrinsic properties of transverse-spatial modes, enables new measurement schemes with sensitivities beyond the classically allowed limits and opens up possibilities to investigate the behaviour of different quantum states.