Nonlinear damping, the change in damping rate with the amplitude of oscillations plays an important role in many electrical, mechanical and even biological oscillators. In novel technologies such as carbon nanotubes, graphene membranes or superconducting resonators, the origin of nonlinear damping is sometimes unclear. This presents a problem, as the damping rate is a key figure of merit in the application of these systems to extremely precise sensors or quantum computers. Through measurements of a superconducting resonator, we show that from the interplay of quantum fluctuations and the nonlinearity of a Josephson junction emerges a power-dependence in the resonator response which closely resembles nonlinear damping. The phenomenon can be understood and visualized through the flow of quasi-probability in phase space where it reveals itself as dephasing. Crucially, the effect is not restricted to superconducting circuits: we expect that quantum fluctuations or other sources of noise give rise to apparent nonlinear damping in systems with a similar conservative nonlinearity, such as nano-mechanical oscillators or even macroscopic systems.

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Microwave optomechanical circuits have been demonstrated to be powerful tools for both exploring fundamental physics of macroscopic mechanical oscillators, as well as being promising candidates for on-chip quantum-limited microwave devices. In most experiments so far, the mechanical oscillator is either used as a passive element and its displacement is detected using the superconducting cavity, or manipulated by intracavity fields. Here, we explore the possibility to directly and parametrically manipulate the mechanical nanobeam resonator of a cavity electromechanical system, which provides additional functionality to the toolbox of microwave optomechanics. In addition to using the cavity as an interferometer to detect parametrically modulated mechanical displacement and squeezed thermomechanical motion, we demonstrate that this approach can realize a phase-sensitive parametric amplifier for intracavity microwave photons. Future perspectives of optomechanical systems with a parametrically driven mechanical oscillator include exotic bath engineering with negative effective photon temperatures, or systems with enhanced optomechanical nonlinearities.

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In this thesis, the microwave detection of mechanically compliant objects is investigated. This starts with a system of a suspended metal drum capacitively coupled to a high impedance microstrip resonator. The mechanical non-linear dissipation of the drums is studied. Next, a suspended nanowire coupled to a CPW resonator is studied. With an electrostatic drive at twice the mechanical resonance frequency, there occurs a parametric excitation of either the mechanical signal or the coupled microwave resonance frequency of the cavity. Then the microwave loss in flux-tunable resonators is investigated for future experiments. One of the goals of this project was to couple a suspended nanowire with a SQUID loop of a flux tunable cavity. Here, the dielectric loss in flux tunable resonators is studied in order to optimize the design of future devices. @en

We experimentally investigate dissipation in mechanical resonators made of a disordered superconducting thin film of a Molybdenum-Rhenium(MoRe) alloy. Electrostatically driving the drum with a resonant AC voltage, we detect its motion using a superconducting microwave cavity. From the temperature dependence of mechanical resonance frequencies and quality factors, we find evidence for non-resonant, mechanically active two-level systems (TLSs) limiting its quality factor at low temperature. In addition, we observe a strong suppression of mechanical dissipation at large mechanical driving amplitudes, suggesting an unconventional saturation of the non-resonant TLSs. These observations shed light on the mechanism of mechanical damping in superconducting drums and routes towards understanding dissipation in such devices.

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