Cryogenic Electronics for Quantum Computer Interface: Low Temperature D/A Converters for Silicon Quantum Computer Controller Circuit

The quest to harness exceptionally high speed computational power exploits the cryogenic Silicon (Si) CMOS technology to find its application in one of the world’s extraordinary research works — ‘Si Quantum Computer’. Silicon solid state quantum bits (qubits) are the fundamental processing elements for these computers. The recent advancements on qubits realization in Si technology indicate that a full-scale (FS) quantum computer will be built in the not-so-distant future. The operating temperature of these qubits are typically ≤ 500mK and they require a cryogenic controller as well as a signal generator circuit which can produce hundreds of different small amplitude fast triggering control signals for their initialisation to start the quantum computation.In this thesis we explore the feasibility of 4.2K (liquid Helium temperature) CMOS data converters to work it as a cryogenic waveform generator to produce the necessary stimuli for the qubits. We choose current steering (CS) D/A converters (DACs) as a potential candidate for this application due to its innate high speed capability of driving resistive loads without the help of any output buffer. However, the lack of a reliable matured cryogenic MOS model makes it difficult to predict the functionality, characteristics and performance of a mismatch constrained CS DAC at 4.2K temperature. Nevertheless, in our first prototype we present a 10 bit partial segmented (4LSB+6MSB) current steering D/A converter in 0.5µm Silicon-on-Sapphire (SOS) CMOS technology which is able to retain its complete monotonic linear behaviour from room to liquid Helium (He) temperature.A low temperature current cell is the building block of a 4.2K CS DAC, and in our second design we develop a unique unit current cell structure to realize a second cryogenic D/A converter. Characterization of the cell through the realization of a 6 bit segmented (4LSB+2MSB) current steering DAC proofs its robustness against the unforeseen extended ‘cryo-effects’ down at liquid He temperature. Both of these converters ability to maintain their full accuracy down at 4.2K temperature opens the door of utilizing the CS DACs as a low temperature signal generator in the future scalable Si quantum computer controller circuit