Master thesis defense by Oliver J. N. Dylander (16/06)

This thesis investigates radio frequency multiplexing as a method for reducing the experimental overhead in cryogenic characterisation of semiconductor spin qubit building blocks. The work addresses the bottleneck created by limited RF access to dilution refrigerators and the time constraints on repeated cooldown cycles. As a foundation for a multiplexed RF printed

circuit board design, a test PCB was characterised through measurements at room temperature and in liquid nitrogen. The room temperature and liquid nitrogen measurements show that the tested shielding configurations, such as copper isolation and guard vias, have a limited influence on averaged RF transmission behaviour and crosstalk between neighbouring traces

in the tested structures. They also show that transition vias lead to larger trace variation, lower transmission and significantly stronger crosstalk.

 

Based on these results and an existing sample PCB, a multiplexing PCB was designed in Altium. This design uses eight switches to route eight RF input lines to four sample spaces, each supporting eight RF lines. The MUX PCB was sent for fabrication, but was not experimentally characterised within the time frame of this thesis.

 

Cryogenic measurements in a Bluefors LD400 show that the measured attenuation of the RF path and the sample PCB is dominated by flex cable contributions rather than by the sample PCB itself. Measurements performed in a Proteox MX are used to characterise the full RF path through a QBoard-II setup, and show that the tank-circuit resonances remain visible at 4 K,

although with shifted frequency positions, broader dips and higher overall attenuation from the cryogenic measurement path.