  We discuss the excitation of polaritons---strongly-coupled states of light
and matter---by quantum light, instead of the usual laser or thermal
excitation. As one illustration of the new horizons thus opened, we introduce
Mollow spectroscopy, a theoretical concept for a spectroscopic technique that
consists in scanning the output of resonance fluorescence onto an optical
target, from which weak nonlinearities can be read with high precision even in
strongly dissipative environments.
