  Electromagnetically induced transparency (EIT) has been realized in atomic
systems, but fulfilling the EIT conditions for artificial atoms made from
superconducting circuits is a more difficult task. Here we report an
experimental observation of the EIT in a tunable three-dimensional transmon by
probing the cavity transmission. To fulfill the EIT conditions, we tune the
transmon to adjust its damping rates by utilizing the effect of the cavity on
the transmon states. From the experimental observations, we clearly identify
the EIT and Autler-Townes splitting (ATS) regimes as well as the transition
regime in between. Also, the experimental data demonstrate that the threshold
$\Omega_{\rm AIC}$ determined by the Akaike information criterion can describe
the EIT-ATS transition better than the threshold $\Omega_{\rm EIT}$ given by
the EIT theory.
