  When one of the parameters in the Euler-Lagrange equations of motion of a
system is modulated, particles can be generated out of the quantum vacuum. This
phenomenon is known as the dynamical Casimir effect, and it was recently
realized experimentally in systems of superconducting circuits, for example by
using modulated resonators made of coplanar waveguides, or arrays of
superconducting quantum intereference devices (SQUIDs) forming a Josephson
metamaterial. In this paper, we consider a simple electrical circuit model for
dynamical Casimir effects, consisting of an LC resonator, with the inductor
modulated externally at 10.8 GHz and with the resonant frequency tunable over a
range of $\pm$ 400 MHz around 5.4 GHz. The circuit is analyzed classically
using a circuit simulator (APLAC). We demonstrate that if an additional source
of classical noise couples to the resonator (on top of the quantum vacuum), for
example via dissipative "internal modes", then the resulting spectrum of the
photons in the cavity will present two strongly asymmetric branches. However,
according to the theory of the dynamical Casimir effect, these branches should
be symmetric, a prediction which is confirmed by our experimental data. The
simulation presented here therefore shows that the origin of the photons
generated in our experiment with Josephson metamaterials is the quantum vacuum,
and not a spurious classical noise source.
