Abstract:

We report on microwave optomechanics measurements performed on a nuclear adiabatic demagnetization cryostat, whose temperature is determined by accurate thermometry from below 500 μK to about 1 Kelvin. We describe a method for accessing the on-chip temperature, building on the blue-detuned parametric instability and a standard microwave setup. The capabilities and sensitivity of both the experimental arrangement and the developed techniques are demonstrated with two different devices: a doubly-clamped beam [1] and a drumhead resonator [2]. We report on an unstable intrinsic driving force in the coupled microwave-mechanical system acting on the mechanics that appears at (very) low temperatures. The origin of this phenomenon remains unknown, and deserves theoretical input. It prevents us from performing reliable experiments below typically 10-30 mK; however no evidence of thermal decoupling is observed, and we show that this feature is present in both devices sharing the same microwave technology, at different levels of strengths. We speculate how the mechanism could be linked to atomic-scale two level systems. Beyond the Stokes sideband pumping instability, we report on self-sustained oscillations for both devices. We analyze the results in the framework of an extended optomechanical theory including Duffing-type and higher-order coupling nonlinear terms. Quantitative understanding of this regime is reached for the drumhead, and should enable its use as a new resource for microwave electronic circuits and quantum non demolition measurements. The described microwave/microkelvin facility is part of the EMP platform [1], and shall be used for further experiments within and below the millikelvin range.