Milli-Kelvin Electronics at the Quantum-Classical Interface

Abstract

The primary research topic is the design of readout circuits for quantum systems at cryogenic temperatures. The work is divided into 3 parts. The first part addresses the modelling of the I-V characteristics of the SiGe HBT over a wide range of temperatures. I empirically prove that the logarithmic slope of the collector current as a function of base-emitter bias is linearly dependent on the y-intercept over the temperature range from 300 K to 6 K. The forward active characteristics at different temperatures can be extrapolated to intersect at a single point. This point is labelled by its temperature-invariant voltage that is predicted to be very close to the bandgap potential at the junction. The second part focuses on the scalability of on-chip readout of semiconductor qubits. I analyze the performance characteristics of a low-power common-emitter transimpedance amplifier. I simulate the electrical behaviour of the amplifier with 70 mK SiGe HBT literature data to understand the achievable fidelity and bandwidth of the readout. The analysis shows that sharper scaling of the transistor characteristics down to the mK range is required to lower the noise temperature of the amplifier below 1 K. I also explore the thermal ramifications of heat generation on the temperature of qubits. The results show a relation between readout circuit integration density and the qubit temperature. Lastly, I present my work on designing, fabricating, and testing the QCPA for the purposes of amplifying qubit readout signals. The amplifier uses the capacitance between a metallic gate and the 2DEG in a GaAs/AlGaAs heterostructure as a medium of frequency mixing resulting in parametric amplification. The paramp, fabricated with the same semiconductor material and processing steps as qubits in GaAs, provides an on-chip, low-noise, wide dynamic range, and magnetically robust method for amplification at mK temperatures.