Master's Defense: Rune Kutchinsky

Complete Design, Implementation and Characterization of an Experiment with a 3D Transmon

A cryogenic experiment with a 3D cavity resonator made from high purity aluminium, a transmon shaped object fabricated in the NBI cleanroom, and a coaxial line setup optimized for high readout fidelity were all designed, made and characterized. The lines were found to be well behaved at temperatures down to 10 mK, with good transmission in the passband, isolation on the readout line, and a TWPA (Travelling Wave Parametric Amplifier) with moderate amplification, the implementation of which effectively implemented real-time control of the experiment. The cavity resonator turned superconducting at low temperature and magnetic field, and though the characterization of the cavity itself turned out to be flawed by overcoupling below Tc, the eigenmodes and quality factors were consistent with literature [1, 2]. The transmon significantly and unexpectedly shifted the TE101 mode up by around 900 MHz, making characterization difficult, though punch out-like behaviour was observed. Subsequent attempts to observe Rabi oscillations remained unsuccessful, either because the transmon was too large for the approximation that the dominant coupling to the cavity is as a dipole, or due to issues with side beads in the fabrication. New, smaller transmon designs, mitigating the side bead issue, and optimized for strong dispersive coupling, are proposed but not implemented. Overall, the entire system was characterized, though much more work would be required to get to the bleeding edge of research.