Master thesis defense by Anton Bagger

Applying alternating bias assisted annealing to reduce the resistance spread of Josephson junctions for superconducting qubits

 The viability of superconducting qubits as a platform for quantum computing is limited by the scale at which qubits can be fabricated with high yield. Yield falls as qubit transition frequencies overlap, and state-of-the-art fabrication still produces junction resistance spreads on the order of a few percent, causing collisions. Motivated by the alternating-bias assisted annealing protocol recently reported by Pappas et al., voltage-based annealing of Al/AlOx/Al junctions was implemented at the Novo Nordisk Foundation Quantum Computing Programme. Alternating-Bias Assisted Annealing (ABAA) protocols described by Pappas et al. and Križan et al. were implemented and evaluated, and two new AC-based protocols, AC Pulse and AC Slow Kicking, were developed. Using AC Pulse at 60 °C, a mean resistance increase above 12.8% was reached with a mean active annealing time of 164 s and without junction breakdown. Heating junctions from 35 °C to 60 °C decreased the measured resistance by up to 14%, motivating protocols viable near ambient temperature. An initial resistance spread of 1.52% across 95 junctions was reduced to 0.52% and held over 51 days. AC Slow Kicking at 35 °C held a spread at or below 0.65% across 49 junctions over two weeks. The resistance of 96 annealed junctions was monitored against unannealed controls in a preliminary study of aging, finding no correlation between drift amplitude and the fractional resistance change induced by annealing. The same study showed a grouping in drift amplitude based on junction design area. Two annealed single-junction qubits showed no reproduction of the coherence improvement reported by Pappas et al. Measurement quality issues were shown to be present for both annealed and control qubits, affecting result interpretation. In the future, the work presented here could be complemented by cryogenic measurements of the critical current of annealed junctions. This would validate results presented here and enable the calibration studies necessary to target specific frequency targets.