Characterizing an itinerant microwave Fock state compatible with transfer to a macroscopic mechanical oscillator*

Transferring propagating single-photon signals generated by a qubit to a mechanical oscillator offers a way to prepare non-classical motional states of a macroscopic object. In this concept, a highly coherent transmon qubit in a cavity is used to create single itinerant microwave photons. These photons can then be directed towards a tunable electromechanical circuit where they can be converted into single phonons. In this talk, we present measurements of itinerant single photons engineered to realize this concept. In particular, we: characterize their quantum state tomographically, demonstrate that they have sufficiently narrow bandwidth for capture by an electromechanical circuit, and measure the efficiency with which they travel between microwave cavities.