Towards cavity quantum circuit electromechanics with millimiter-sized silicon nitride membranes

Many everyday use appliances consist of hybrid setups, i.e., devices that rely on a close-knit interfacing between different physical elements, each of which can carry out a given task in a complementary way. Interfaces with mechanical membranes and superconducting circuits play an active role in today's research towards a new generation of hybrid devices, the dynamics of which may be prominently ruled by quantum mechanics. Crucial to succesfully harness the quantized dynamics of such devices is their isolation from environmental noise sources. For the millimiter sized membranes we use in our circuits this even requires isolation from the acoustic noise present in the dilution refrigerators wherein they are usually hosted. Based on a systematic characterization of the noisy acoustic signals disturbing our electromechanical circuit we design different methods to suppress them, including an original mass-spring system that enables keeping our device freely suspended inside the fridge. Using these adjustments, and a phononic bandgap shield to minimize acoustic radiation loses of the membrane itself, we prove feasible ground state cooling of the membrane's fundamental mode of vibration.