  Quantum information is typically encoded in the state of a qubit that is
decoupled from the environment. In contrast, waveguide quantum electrodynamics
studies qubits coupled to a mode continuum, exposing them to a loss channel and
causing quantum information to be lost before coherent operations can be
performed. Here we restore coherence by realizing a dark state that exploits
symmetry properties and interactions between four qubits. Dark states decouple
from the waveguide and are thus a valuable resource for quantum information but
also come with a challenge: they cannot be controlled by the waveguide drive.
We overcome this problem by designing a drive that utilizes the symmetry
properties of the collective state manifold allowing us to selectively drive
both bright and dark states. The decay time of the dark state exceeds that of
the waveguide-limited single qubit by more than two orders of magnitude.
Spectroscopy on the second excitation manifold provides further insight into
the level structure of the hybridized system. Our experiment paves the way for
implementations of quantum many-body physics in waveguides and the realization
of quantum information protocols using decoherence-free subspaces.
