  We have developed and tested a doubly tunable resonator, with the intention
to simulate fast motion of the resonator boundaries in real space. Our device
is a superconducting coplanar-waveguide half-wavelength microwave resonator,
with fundamental resonant frequency ~5 GHz. Both of its ends are terminated by
dc-SQUIDs, which serve as magnetic-flux-controlled inductances. Applying a flux
to either SQUID allows tuning of the resonant frequency by approximately 700
MHz. By using two separate on-chip magnetic-flux lines, we modulate the SQUIDs
with two tones of equal frequency, close to twice that of the resonator's
fundamental mode. We observe photon generation, at the fundamental frequency,
above a certain pump amplitude threshold. By varying the relative phase of the
two pumps we are able to control the photon generation threshold, in good
agreement with a theoretical model for the modulation of the boundary
conditions. At the same time, some of our observations deviate from the
theoretical predictions, which we attribute to parasitic couplings, resulting
in current driving of the SQUIDs.
