Multi-photon dressing of an anharmonic superconducting many-level   quantum circuit
  We report on the investigation of a superconducting anharmonic multi-level circuit that is coupled to a harmonic readout resonator. We observe multi-photon transitions via virtual energy levels of our system up to the fifth excited state. The back-action of these higher-order excitations on our readout device is analyzed quantitatively and demonstrated to be in accordance with theoretical expectation. By applying a strong microwave drive we achieve multi-photon dressing within our anharmonic circuit which is dynamically coupled by a weak probe tone. The emerging higher-order Rabi sidebands and associated Autler-Townes splittings involving up to five levels of the investigated anharmonic circuit are observed. Experimental results are in good agreement with master equation simulations. 
Jochen Braum\"uller, Joel Cramer, Steffen Schl\"or, Hannes Rotzinger,   Lucas Radtke, Alexander Lukashenko, Ping Yang, Sebas...
cond-mat.mes-hall cond-mat.supr-con quant-ph