  We have observed the Autler-Townes doublet in a superconducting Al/AlOx/Al
transmon qubit that acts as an artificial atom embedded in a three-dimensional
Cu microwave cavity at a temperature of 22 mK. Using pulsed microwave
spectroscopy, the three lowest transmon levels are isolated, eliminating
unwanted effects of higher qubit modes and cavity modes. The long coherence
time (~40 us) of the transmon enables us to observe a clear Autler-Townes
splitting at drive amplitudes much smaller than the transmon level
anharmonicity (177 MHz). Three-level density matrix simulations with no free
parameters provide excellent fits to the data. At maximum separation, the
fidelity of a dark state achieved in this experiment is estimated to be
99.6-99.9%.

Quantum Physics;Mesoscale and Nanoscale Physics;Atomic Physics