Post-fabrication frequency tuning in superconducting transmon qubits
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Övrigt, MSc
Publicerad
Författare
Toselli, Maurizio
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
This thesis presents a study on the manipulation of the normal state resistance
RN of Al/AlOx/Al Josephson junctions using electrical DC signals. The goal is to
develop a post-fabrication method for tuning the frequency of transmon qubits in
superconducting quantum processors. This method aims to correct initial fabrication uncertainties and mitigate frequency collisions to scale up the number of qubits.
The project initially focused on a precise, non-intrusive technique to measure RN
at room temperature, which is directly related to the qubit plasma frequency f01
through the Ambegaokar-Baratoff relation. Subsequently, the natural aging of two
types of devices, thin-oxide and thick-oxide junctions, was studied. Thin-oxide junctions proved less stable, exhibiting a resistance increase of more than 30% within
the first two weeks after fabrication, compared to the 3% to 4% observed in thickoxide junctions. A procedure was then developed to deliberately increase RN at
room temperature using high DC voltage biases, achieving increases of nearly 20%
for thin-oxide devices and about 10% for thick-oxide ones. This demonstrates the
potential for correcting fabrication variations on a wafer-level scale. An essential
finding was the delayed resistance increase after manipulation, suggesting the need
for further studies to better understand and control this effect. Theoretical modeling
and simulations also revealed that the applied voltage plays a crucial role beyond
simply delivering localized heat to the junction. Finally, a technology demonstration
performed with a new experimental setup specifically designed to address individual
qubits validated this method on a working quantum processor, providing a proof
of concept. The results showed frequency shifts of several hundred megahertz after
manipulation, without compromising qubit lifetimes. Despite a systematic overestimation of the final frequencies, likely due to the delayed resistance increase, this
thesis successfully demonstrates a promising frequency tuning method and suggests
future optimization and implementation directions.
Beskrivning
Ämne/nyckelord
frequency tuning method, transmon qubit, Josephson junction, super conducting quantum computing, electrical measurements, junction aging