  We experimentally 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, using digital filtering. For an optimized
temporal filter, we observe a large Wigner logarithmic negativity, in excess of
0.08, in agreement with theory. The fidelity between the theoretical
predictions and the states generated experimentally is up to 99%, reaching
state-of-the-art realizations in the microwave frequency domain. Our results
provide a new way to generate and control nonclassical states, and may enable
promising applications such as quantum networks and quantum computation based
on waveguide quantum electrodynamics.
