Propagating Wigner-negative states from the steady-state emission of a superconducting qubit

Nonclassical states of the electromagnetic field are an essential resource for quantum communication and quantum computation. Specifically, to obtain a quantum computational advantage it is necessary to have states that are nonclassical in the sense that they have a negative Wigner function. In circuit quantum electrodynamics setups, such nonclassical states have been generated in confined cavity modes. Propagating Wigner-negative states have been experimentally produced by either releasing a cavity state into a waveguide, or by pulsing a quantum system such as a qubit. Here, by contrast, we demonstrate the steady-state generation of propagating Wigner-negative states at steady state, from a continuously driven superconducting qubit. We reconstruct the Wigner function of the radiation emitted into various propagating modes, defined by their temporal envelopes, using linear amplifiers and digital filtering. We observe a large Wigner logarithmic negativity, in excess of 0.08, for an optimized temporal envelope, in agreement with theory. Our results provide an alternative route to engineer nonclassical states useful for quantum computation.