Design of a variable coupler for microwawe-optical quantum transduction

This thesis work is the result of activities carried out at Università di Pisa and the traineeship at the Superconducting Radio-Frequency (SRF) department of Fermilab, America’s premier laboratory for particle physics and accelerator research. The superconducting qubits that are used in a quantum computer operate at millikelvin temperature in a dilution refrigerator, and they have their quantum states encoded in microwave photons, transfering that microwave quantum information out of the refrigerator to a different quantum system would swamp the signal with thermal noise when emerging to room temperature. A solution is to use a quantum transducer that can convert the quantum signals from microwave to optical, and send quantum information through fiber optics cables. There are a few different approaches to quantum transduction, but the Superconducting Quantum Materials and Systems Center (SQMS) is focusing on an electrooptic quantum transducer for microwave-to-optical frequency conversion using a lithium niobate WGM resonator and a diamond coupling prism. Since the resonator system’s coupling strength is tunable by changing the distance between the diamond prism and the resonator, this work has the purpose of developing a mechanical design for a variable optical coupler which works in a dilution refrigerator at 10 milli-kelvin. This device will allow a micro-positioning of the prism with respect to the resonator, to achieve the maximum optical coupling between the two. In order to do that, the paper proposes: • An experimental validation of the optical model to understand how many degrees of freedom will be needed to the prism with a test bench at room temperature. • The design of the mechanical device, needed to perform the micro-positioning in operation with analysis using finite element software. • Thermal analysis of the superconducting radio-frequency (SRF) cavity. At the end of the analysis, the thesis illustrates the results of the research and work activity, highlighting the performances of this new mechanical device which is a big step in realizing a transducer that can be exploited for high-fidelity heralded quantum entanglement generation between two distant quantum processing units, as well as for high-precision microwave signal measurement and quantum sensing