  Vacuum modes confined into an electromagnetic cavity give rise to an
attractive interaction between the opposite walls. When the distance between
the walls is changed non-adiabatically, virtual vacuum modes are turned into
real particles, i.e. photons are generated out of the vacuum. These effects are
known as the static and dynamical Casimir effect, respectively. Here we
demonstrate the dynamical Casimir effect using a Josephson metamaterial
embedded in a microwave cavity at 5.4 GHz. We achieve the non-adiabatic change
in the effective length of the cavity by flux-modulation of the SQUID-based
metamaterial, which results in a few percent variation in the velocity of
light. We show that energy-correlated photons are generated from the ground
state of the cavity and that their power spectra display a bimodal frequency
distribution. These results are in excellent agreement with theoretical
predictions, all the way to the regime where classical parametric effects
cannot be of consequence.
