Thesis presented January 13, 2023

Abstract:
Thousands to millions of sensitive signals will need to be conveyed through all the temperature stages of a dilution fridge to operate future large-scale quantum processors made of many quantum bits. The exploding number of heat-conductive coaxial cables will overwhelm the fridge cooling capabilities, detrimental to the quantum core. Moving the control electronics down to cryogenic temperatures allows the use of already-available superconducting cables, alleviating heat conduction between low-temperature stages, and appears as a clear path towards scalability in the number of operating qubits.
This Ph.D. work aimed to explore the use of the industrial CMOS 28nm Fully-Depleted Silicon-On-Insulator (FD-SOI) technology at cryogenic temperatures for quantum computing applications. Our first objective is to extend the sparse existing knowledge about the FD-SOI 28nm transistors at cryogenic temperatures for practical aspects of circuit design and later for developments of compact models.
To speed-up the characterization of single devices with the inherent hour-long cooling cycles, we designed an integrated circuit multiplexing a thousand of transistors for different geometries and gate-stack flavors for low-frequency measurement of the current-voltage characteristics and pair-matching analysis from 300 down to 0.1K. We discuss and analyze the evolution trends with varying geometry at different temperatures for important quantities in circuit design, such as the transconductance, the conductance, and the transconductance over drain current ratio of individual transistors.
Secondly, we explore the low-temperature co-integration and the full on-chip integration of semiconductor quantum devices with classical electronics aimed at specific measurements down to the millikelvin range.
We first focus on the sub-nanoampere current measurement of quantum dot devices by designing and characterizing a low-power transimpedance amplifier (TIA). The high-gain amplifier is successfully applied to measure the current across single- and double quantum-dot devices, respectively wire-bonded a few mm away or on-chip integrated a few micrometers-away. To further leverage the integration into the same substrate, we connected GHz-range voltage-controlled oscillators to one of the gates of the double dot in an attempt to observe discrete charge pumping in a fully-integrated device.
Lastly, we tackled the measurement of the gate capacitance of single quantum devices by proposing a new measurement scheme making use of the cryogenic electronics capabilities as an alternative to the well-known reflectometry. By integrating a voltage-controlled current excitation and a voltage-sensing amplifier in the 200 MHz range, both in close proximity to the quantum device connected to an LC tank, the read-out circuitry of a variation in the device capacitance becomes a purely lumped-element system with impedance measurement of the resonant circuit without any wave propagation like in reflectometry. This approach increases simplicity and compactness of the measurement set-up. We even replace the bulky passive inductor used in reflectometry by an active inductor made of transistors and capacitors, offering improved scalability with a 3-orders of magnitude lower area for the same inductance. The resulting circuit successfully measured aF-capacitance variations of nanometric transistors at 4.2K revealing oscillatory quantum effects in the gate capacitance as a function of the gate and back-gate voltages.
At the end of this dissertation, a picture is given with the challenges laying ahead related to circuit architecture and design for the ultimate goal of entering into the era of large-scale quantum processing.

Keywords:
Cryogenics, quantum computing, electronics