Cryogenic SiGe Bipolar Amplifiers for Spin-Qubit DC Readout

Abstract

In recent years, quantum computers have become increasingly popular, with high-profile companies in-vesting more and more resources into the development of quantum-computing prototypes. Such interest is motivated by the exponential increase in the computing power that quantum computers offer over classical computers, which may be used in several practical applications, such as producing faster and more accurate solutions to machine learning algorithms, the quantum simulations of molecules for drug and material synthesis, techniques that combat cybersecurity threats, and more. Quantum processors are typically placed in a dilution refrigerator, which cools down the processor to near absolute zero to make use of their quantum behavior. These fridges are large in scale and require multiple long wires from the processor to the readout/control equipment, which mostly resides at room temperature. However, although this approach is suitable for currently available systems with few quantum bits (qubits), it is unpractical for future quantum computers, with thousands or millions of qubits, due to the interconnect bottleneck. One solution isto move the readout/control circuits closer to the processor to avoid the need for bulky unreliable interconnects. However, this has proven to be difficult due to the limited cooling power of the fridges, which is below a few Watts at 4 K and below a few mW at temperatures below 100 mK. These limitations stress the need for low power readout/control circuitry of the processor as the number of qubits in a single processor increases. This thesis focuses on an ultra-low-power cryogenic amplifier design for the readout of spin qubits.Only recently, the cryogenic performance of Silicon-Germanium Heterojunction Bipolar Transistors (SiGe HBT) has been demonstrated to be specifically suited for the cryogenic readout of spin qubits, because of their ultra-low-power and their high sensitivity. This thesis focuses on the standard SiGe HBT available in the IHP 0.13μm SG13G2 BiCMOS process and on an optimized variant with modified doping profiles for enhanced current gain. As a first step, a comprehensive cryogenic (4 K or 15 K) DC characterization of the SiGe HBTs, CMOS transistors, and resistors is performed. This data proves the suitability of such technology for cryogenic application and will enable the future integration of a complete qubit readout, including SiGe front-end amplification and CMOS back-end post-processing. The measurement results of the SiGe HBT characterization show outstanding cryogenic performance for low power applications, with an attain-able current gain in the standard HBT of 100, 500, and 1000 at a power dissipation of 61nW, 2μW and 30μW,respectively, and, for the modified HBT, of 100, 500 and 1000 at a power dissipation of 5.5nW, 90nW, and140nW, respectively. To mimic the spin qubit readout, a discrete amplifier employing a SiGe HBT produced in the IHP SG13G2process has been built and used to characterize a single-electron transistor (SET), with both the SET and the amplifier operating at 4 K. By comparing the charge-stability diagram measured with room-temperature equipment, the proper operation of the proposed amplifier is demonstrated. Furthermore, the amplifier characterization showed a gain > 40 dB with an 113 kHz -3-dB bandwidth, and Signal-Noise Ratio (SNR)above 10 dB for a total measurement time of t<10μs, thus demonstrating performance compatible with the requirements for single-shot readout in state-of-the-art spin-qubit computers. Overall, the results show the capabilities of the SiGe HBT in low power cryogenic applications as well as the capabilities of the 0.13μmBiCMOS technology for the future full integration of qubit readout schemes.