For this project, a series of devices have been measured, aimed at uncovering the first necessary ingredient for any fusion experiment: the quasiparticle poisoning time of the superconducting system. This time provides an upper bound for the timescale within which any fusion experiment needs to be performed.
To characterize the quasiparticle poisoning time, I have designed and measured a series of quantum-dot devices, where the quantum dot is a semiconductor in close proximity to a superconductor. The initial goal was to observe an even-odd periodicity in the Coulomb-peak spacing. This is a telltale sign of induced superconductivity. Of all the devices measured, some devices exhibited even-odd periodicity, and sometimes even 2e periodicity, whereas the majority did not. Of the devices that exhibit even-odd periodicity, it was only observed in very specific configurations. This was in strong contrast with measurements performed on proximitized nanowires, where even-odd periodicity and 2e periodicity was observed over large configuration ranges. One likely explanation for this discrepancy is that InAs 2DEG might not have a hard superconducting gap, and as a result acts as a quasiparticle trap.
Focusing on the devices that showed the clearest even-odd periodicity, I then proceeded to characterize the quasiparticle poisoning times. To this end, I created an rf reflectometry setup together with a charge detector (a single electron transitor) to detect quasiparticle poisoning events at a high sampling rate (<1 usec). I was able to measure low quasiparticle poisoning times (>100 msec) when a quantum dot was fully isolated from its environment, as would be expected. However, upon increasing the tunnel coupling to the superconductor, the quasiparticle poisoning rates rapidly increased to below the sampling rate (<1 usec). This was observed for all devices measured. This is in stark contrast with similar measurements performed on InAs nanowires, where the quasiparticle poisoning rate remained low (> 1 msec) even after strongly coupling the quantum dot to a neighbouring superconducting lead. Given that the measured quasiparticle poisoning times in the 2DEG are beyond the measurement resolution, there was no way to proceed with a fusion experiment. Changes in fabrication routines and device geometries were not able to improve the quasiparticle poisoning times sufficiently.
