  Isolation of a system from its environment is often desirable, from precision
measurements to control of individual quantum systems; however, dissipation can
also be a useful resource. Remarkably, engineered dissipation enables the
preparation of quantum states of atoms, ions or superconducting qubits as well
as their stabilization. This is achieved by a suitably engineered coupling to a
dissipative cold reservoir formed by electromagnetic modes. Similarly, in the
field of cavity electro- and optomechanics, the control over mechanical
oscillators utilizes the inherently cold, dissipative nature of the
electromagnetic degree of freedom. Breaking from this paradigm, recent
theoretical work has considered the opposite regime in which the dissipation of
the mechanical oscillator dominates and provides a cold, dissipative reservoir
to an electromagnetic mode. Here we realize this reversed dissipation regime in
a microwave cavity optomechanical system and realize a quasi-instantaneous,
cold reservoir for microwave light. Coupling to this reservoir enables to
manipulate the susceptibility of the microwave cavity, corresponding to
dynamical backaction control of the microwave field. Additionally, we observe
the onset of parametric instability, i.e. the stimulated emission of microwaves
(masing). Equally important, the reservoir can function as a useful quantum
resource. We evidence this by employing the engineered cold reservoir to
implement a large gain (above 40 dB) phase preserving microwave amplifier that
operates 0.87 quanta above the limit of added noise imposed by quantum
mechanics. Such a dissipative cold reservoir forms the basis of microwave
entanglement schemes, the study of dissipative quantum phase transitions,
amplifiers with unlimited gain-bandwidth product and non-reciprocal devices,
thereby extending the available toolbox of quantum-limited microwave
manipulation techniques.
