  We have studied both theoretically and experimentally how an LC series
circuit connected in parallel to a Josephson junction influences the Josephson
dynamics. The presence of the shell circuit introduces two energy scales, which
in specific cases can strongly differ from the plasma frequency of the isolated
junction. Josephson junctions were manufactured using Nb/Al-AlOx/Nb fabrication
technology with various on-chip LC shunt circuits. Spectroscopic measurements
in the quantum limit show an excellent agreement with theory taking into
account the shunt inductance and capacitance in the Resistively and
Capacitively Shunted Junction model. The results clearly show that the dynamics
of the system are two-dimensional, resulting in two resonant modes of the
system. These findings have important implications for the design and operation
of Josephson junctions based quantum bits.
