PhD Defense: Yannick Seis

Ultra-Coherent Electro-Mechanics in the Quantum Regime

Over the last twenty years, the study of mechanical oscillators for sensing and quantum applications has gained great interest. The versatility of possible couplings of mechanics to other system such as optical, microwave, solid-state spin systems, all of which can already be operated in the quantum regime, makes them important candidates for hybrid interconnects in quantum devices and networks.
In this work we consider one such interface between a mechanical oscillator and a microwave circuit. The fragility of quantum states to their environment necessitates the development high-quality materials and devices to preserve the quantum nature of the studied interactions.
Our group has pioneered ultra-coherent mechanical oscillators which we here integrate into a low-loss superconducting resonator such that mehanical motion can be manipulated and read-out by microwave light.
We describe the cleanroom fabrication flow of a NbTiN superconducting resonator, an Al-metallised patterned SiN membrane and their assembly into a flip-chip electro-mechanical device. Then we characterise microwave, mechanical and interaction parameters at millikelvin
temperatures: with mechanical quality factor of 1.5 · 10^9 at 2π · 1.5 MHz, deep sideband resolution with total cavity decay rate of 2π · 230 kHz and modest single-photon coupling of 2π · 0.87 Hz. The main result of this work is the cavity-assisted cooling of the mechanics into its quantum ground state of motion, that is with an average occupancy less than a single quantum.
Ultra-coherent mechanical oscillators integrated into quantum devices can serve as long-lived memories for quantum states manipulated in superconducting qubits. A simultaneous interfacing of mechanical elements with microwave and optical resonators can establish electro-opto-mechanical transduction between superconducting quantum computers and long-distance optical communication networks.