The Development of a Scalable Atomic Quantum Integrated Circuit

Abstract

Silicon spin qubits are strong candidates in the efforts to build a universal quantum computer. They can be fabricated on the nanometer scale, have low noise and long coherence times. To date, efforts for fabricating donor quantum dots in silicon have only focused on two qubits or less. Presented in this thesis is the design, fabrication, and measurement of a twenty-quantum dot Si:P device, the basis of a 10-qubit singlet-triplet architecture.

To individually address each qubit, we designed and manufactured superconducting NbTiN frequency division multiplexing chips. These chips were demonstrated to be reproducible and could be used to readout up to 20 quantum dots using only one input and one output line, an important step for increasing device scalability. This multiplexing chip was characterised under magnetic field and used to demonstrate single shot readout of the 𝑆−𝑇− state with 90% readout fidelity using in-situ gate-based dispersive readout, the highest fidelity readout to date for this system.

We then fabricated the largest integrated Si:P qubit circuit using scanning tunnelling microscope (STM) lithography to date, five times larger than previous devices. This scale-up required improvements to the manufacturing process, including extension of electron beam lithography capabilities for defining ohmic structures. A custom cryogenic printed circuit board was designed for the device and demonstrated to have low crosstalk of less than -40 dB up to 4 GHz. The device was characterised at millikelvin in a dilution fridge with charge readout performed on 8 of the 10 double dots.

Finally, a scalable measurement system based on the PCI eXtensions for Instrumentation (PXI) platform was created, enabling simultaneous multitone digital signal generation and filtering. This system reduced the hardware requirements of previous analogue setups by 80% and enabled four times better signal to noise (SNR) ratio for charge readout as a result of improved low pass digital filtering. The first demonstration of simultaneous readout of neighbouring qubits in the Si:P platform is presented and used to correlate noise sources in the device showing that noise is strongly correlated across four double dots separated by 300 nm.