PhD Defense: Albert Hertel

Albert Hertel

Development of superconducting gatemon qubits based on selective-area-grown semiconductors

Currently much experimental effort at universities and companies focuses on the development of large scale quantum computers. Quantum computers are believed to enable solving certain computational problems faster than classical computers, thus revolutionizing many fields in science. Many different technologies are competing to overcome challenges in scaling today's small quantum processors to practically useful fault tolerant quantum computers. Superconducting qubits -- in particular transmon-type qubits -- are a leading technology in the field and the subgroup of gate-tunable transmons has recently shown strong potential to become a platform for low crosstalk and low dissipation qubit systems. This thesis presents novel material platforms for scalable voltage-controlled semiconductor-based superconducting transmon qubits (gatemons). These gatemons are based on selective-area-grown InAs/Al hybrid structures which are monolithically integrated into a high resistivity silicon substrate (Si SAG) or InP substrate (InP SAG).

Starting with proof-of-principle demonstrations, the InP SAG material system is introduced and the gatemon fabrication is outlined. Coherent oscillations are demonstrated and coherence times T₁ ~180ns and T₂^* ~10ns are measured. To improve coherence times, an alternative growth sequence  is explored and the electric properties of the material are characterized.

Moving towards gatemons on silicon, the electrical properties of Si SAG at millikelvin temperatures are characterized where we observe a high average field-effect mobility of ~ 3200cm²/Vs for the InAs channel, a hard induced superconducting gap, high Josephson junction transparency of ~0.75 and signatures of multiple Andreev reflections. Josephson junctions exhibit a gate voltage tunable switching current with a product of the critical current and normal state resistance, I_CR_{N, of  ~}83µV.

Finally, we discuss the RF properties of Si SAG and demonstrate that high quality resonators can be fabricated on the silicon substrate. After detailing the gatemon device fabrication, we describe the measurement of coherent oscillations and coherence times T₁ ~380ns and T₂^*~15ns are measured. Possible steps towards increased coherence times are outlined. 

In summary, the work presented in this thesis presents a novel and promising material platform for scalable voltage-controlled qubit circuitry.

Zoom link: https://ucph-ku.zoom.us/j/69301832228