  Quantum two-level systems interacting with the surroundings are ubiquitous in
nature. The interaction suppresses quantum coherence and forces the system
towards a steady state. Such dissipative processes are captured by the
paradigmatic spin-boson model, describing a two-state particle, the "spin",
interacting with an environment formed by harmonic oscillators. A fundamental
question to date is to what extent intense coherent driving impacts a strongly
dissipative system. Here we investigate experimentally and theoretically a
superconducting qubit strongly coupled to an electromagnetic environment and
subjected to a coherent drive. This setup realizes the driven Ohmic spin-boson
model. We show that the drive reinforces environmental suppression of quantum
coherence, and that a coherent-to-incoherent transition can be achieved by
tuning the drive amplitude. An out-of-equilibrium detailed balance relation is
demonstrated. These results advance fundamental understanding of open quantum
systems and bear potential for the design of entangled light-matter states.
