  We propose a protocol able to prepare two remote and initially uncorrelated
microwave modes in an entangled stationary state, which is certifiable using
only local optical homodyne measurements. The protocol is an extension of
continuous variable entanglement swapping, and exploits two hybrid
quadripartite opto-electro-mechanical systems in which a nanomechanical
resonator acts as a quantum interface able to entangle optical and microwave
fields. The proposed protocol allows to circumvent the problems associated with
the fragility of microwave photons with respect to thermal noise and may
represent a fundamental tool for the realization of quantum networks connecting
distant solid-state and superconducting qubits, which are typically manipulated
with microwave fields. The certifying measurements on the optical modes
guarantee the success of entanglement swapping without the need of performing
explicit measurements on the distant microwave fields.
