  Magnetic impurities on superconductors lead to bound states within the
superconducting gap, so called Yu-Shiba-Rusinov (YSR) states. They are parity
protected, which enhances their lifetime, but makes it more difficult to excite
them. Here, we realize the excitation of YSR states by microwaves facilitated
by the tunnel coupling to another superconducting electrode in a scanning
tunneling microscope (STM). We identify the excitation process through a family
of anomalous microwave-assisted tunneling peaks originating from a second-order
resonant Andreev process, in which the microwave excites the YSR state
triggering a tunneling event transferring a total of two charges. We vary the
amplitude and the frequency of the microwave to identify the energy threshold
and the evolution of this excitation process. Our work sets an experimental
basis and proof-of-principle for the manipulation of YSR states using
microwaves with an outlook towards YSR qubits.
