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 platformsfor 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 ≈ 180 ns and T^•_{2 ≈}  10 ns 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 μ ≈ 3200 cm² /�Vs for the InAs channel, a hard induced
superconducting gap, high transparency Josephson junctions T ≈ 0.75 and
signatures of multiple Andreev reflections. Josephson junctions exhibit a gate
voltage tunable switching current with I_CR_N ≈ 83 μV.
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 ≈ 380 ns and T^·₂ ≈ 15 ns are measured.
Possible steps towards increased coherence times are outlined.
In summary, thework presented in this thesis presents a novel and promising
material platform for scalable voltage-controlled qubit circuitry.