Quantum superposition of a single microwave photon in two different ’colour’ states

A single microwave photon is prepared in a superposition of two states of different frequency. This is achieved by using a superconducting quantum interference device to mediate the coupling between two harmonics of a superconducting resonator. Fully controlled coherent coupling of arbitrary harmonic oscillators is an important tool for processing quantum information 1 . Coupling between quantum harmonic oscillators has previously been demonstrated in several physical systems using a two-level system as a mediating element 2 , 3 . Direct interaction at the quantum level has only recently been realized by means of resonant coupling between trapped ions 4 , 5 . Here we implement a tunable direct coupling between the microwave harmonics of a superconducting resonator by means of parametric frequency conversion 6 , 7 . We accomplish this by coupling the mode currents of two harmonics through a superconducting quantum interference device (SQUID) and modulating its flux at the difference (∼7 GHz) of the harmonic frequencies. We deterministically prepare a single-photon Fock state 8 and coherently manipulate it between multiple modes, effectively controlling it in a superposition of two different ’colours’. This parametric interaction can be described as a beamsplitter-like operation that couples different frequency modes. As such, it could be used to implement linear optical quantum computing protocols 9 , 10 on-chip 11 .