Noise Filtering and Optical readout for quantum circuit electromechanics

Abstract

Circuit electromechanics and Josephson junction based superconducting qubits are two experimental fields that have generated vast attention in the past decade. This thesis aims at solving some experimental challanges in dealing with these systems. The phase noise of microwave sources is an issue that plagues these experiments and custom built filter cavities are used to reduce the phase noise by tuning it to a particular frequency. In this work, a system of a filter cavity and two motors was assembled, and a program based on the Nelder-Mead minimisation algorithm was written to tune the filter cavity to any arbitrary frequency by looking at its response on a network analyser. Tuning with the frequency space accuracy of 50 kHz, and extinction of around 20-30 dB is demonstrated. The second challange that was addressed was the readout of weak signals from the superconducting devices at cryogenic temperatures. As the conventionally used HEMT amplifiers and coaxial cables contribute to a significant heat load, we instead implement LiNbO3 electro-optic phase modulator, connected to room temperature heterodyne detection setup by optical fibers of lower thermal conductivity. The characterisation of the phase modulator was done by measuring its half-wave voltage at cryogenic temperatures. Observation of optomechanically induced transparency and parametric instability was demonstrated using both the conventional HEMT readout as well as EOM-based optical readout.