  We report on fast tunability of an electromagnetic environment coupled to a
superconducting coplanar waveguide resonator. Namely, we utilize a
recently-developed quantum-circuit refrigerator (QCR) to experimentally
demonstrate a dynamic tunability in the total damping rate of the resonator up
to almost two orders of magnitude. Based on the theory it corresponds to a
change in the internal damping rate by nearly four orders of magnitude. The
control of the QCR is fully electrical, with the shortest implemented operation
times in the range of 10 ns. This experiment constitutes a fast active reset of
a superconducting quantum circuit. In the future, a similar scheme can
potentially be used to initialize superconducting quantum bits.
