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

Nonclassical states of the electromagnetic field constitute an essential resource for quantum communication and 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 superconducting quantum circuits, Wigner-negative states have been generated in confined cavity and propagating modes. These propagating states are generated by either releasing a cavity state or by exciting a quantum emitter that subsequently decays into a waveguide. In contrast to these pulsed schemes, here we demonstrate the steady-state generation of propagating Wigner-negative states from a continuously driven superconducting qubit. We reconstruct the Wigner function of the radiation emitted into propagating modes defined by their temporal envelopes. For an optimized temporal filter, we observe a substantially increased Wigner negativity in agreement with theory. Our results provide an alternative way to generate and control nonclassical states, and may enable promising applications such as quantum networks and computation based on waveguide quantum electrodynamics.