  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.
