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Optimization of silicon nitride membranes for hybrid superconducting-mechanical circuits

  • Ivanov, Edouard
Publication Date
Feb 22, 2021
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In this thesis, we study an engineered mechanical oscillator coupled to a microwave cavity. In a preliminary experiment, microwave photons were used as a cold bath to reduce the temperature of the mechanical oscillator by a factor 500. We present several improvements to the membranes which should enable us to cool them down to their quantum ground state. In particular, we explore the rich physics of phononic bangaps to isolate an ultrahigh-quality-factor membrane mode from decoherence channels, a technique known as ``soft-clamping''. Using a quantum-limited interferometer able to resolve the membrane's Brownian motion, we reconstruct the profiles of the membrane modes. Thanks to this setup we identify a set of parasitic membrane modes which significantly degrade the quality factor of the soft-clamped modes. Specific mode engineering strategies are therefore implemented to ensure the optimal performance of the softly-clamped membranes. Once integrated in an electromechanical cavity, the optimized membranes developed over the course of this thesis should operate deep in the quantum regime. We discuss the perspectives of preparing nonclassical states of motion by exploiting superconducting qubits as a nonlinear resource. In particular, we propose a scheme which could achieve this using a single microwave-photon detector, developed in collaboration with LPENS. This hybrid electromechanical system could be used to store fragile quantum states on the scale of seconds, to measure minute forces with unprecedented precision, or to study the boundary between the quantum and the classical worlds.

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