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(2022)ThesisIn this thesis we investigate donor molecules as a resource for scaling-up donor-based spin qubits in silicon towards error-corrected quantum computers. We first propose a novel donor-based qubit consisting of an electron spin spread across a single donor (1P) and a two-donor molecule (2P) that is electrically driven and coupled utilising the hyperfine interaction. This qubit belongs to a class of electron spin qubits called “flopping-mode” qubits, where the electron wave function is spread over two quantum dots. Using a complete error model, we first investigate how to minimise errors and the electrical driving power in the general class of qubits by optimising the magnetic gradients across the device. We then demonstrate how the magnetic gradient on the new qubit design can be atomically engineered using the hyperfine interaction within the donor molecules. In particular we show that by controlling the orientation of the nuclear spins in the qubit we can suppress the deleterious magnetic gradient originating from the hyperfine interaction within the two-donor molecule. We predict qubit errors well below 10−3 and show that the 1P-2P flopping-mode qubit can be strongly coupled to a superconducting cavity. Finally, we outline a way to scale the proposed qubits to a larger cavity-based architecture. Following this theoretical proposal we present experimental evidence that the required engineering of 2P donor-molecules is possible, using two atomic precision devices fabricated with hydrogen-resist STM lithography. We achieve high accuracy precision patterning of two different devices that contain donor molecules to reveal the following results. First we show how to improve the fidelity of single shot electron spin readout from 83% to 94% using an optimised SET design. ESR spectra performed using adiabatic spin inversion yield precise measurements of the hyperfine interactions within the molecule. Using atomistic tight binding calculations in collaboration with the group of Professor Rahman we were able to perform metrology of the individual donor configurations within the dots. The metrology could be performed with a precision of ±0.25nm using measurements of the charging energies and improved to atomic precision using the hyperfine spectroscopy. The ESR spectra demonstrated the first observation of a Stark shift in a tightly bound donor molecule. The magnitude of the shift observed was shown to depend on the molecular orientation within the crystal and offers future strategies for hyperfine engineering for optimal qubit operation. Finally, we demonstrate the first nuclear-spin readout of a tightly-bound donor molecule, with a fidelity of 88%, and show how we can track the nuclear spin states over time. Using a hidden Markov model we extracted the nuclear transition frequencies and uncover possible evidence of a dipolar coupling between the nuclear spins.
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(2023)ThesisStrongly correlated materials can display exotic quantum phenomena relevant to the emergence of quantum technologies. Yet, many questions have remained open because of the difficulty in modelling the underlying physics beyond a few lattice sizes. Therefore, a continuing effort has been made to develop quantum simulation platforms, where these many-body states can be artificially emulated with a great degree of control in bespoke hardware implementations. This thesis focuses on the use of dopant-based atomic-scale devices in silicon to build artificial quantum matter probed by a low-temperature scanning tunnelling microscope (STM). We combine STM hydrogen lithography with phosphine doping for the placement of phosphorus donors in our silicon devices with atomic precision and perform spatially resolved measurements using the STM tip. We first establish the experimental platform by characterising strongly correlated states found in single and pairs of quantum dots made of a few donors each. Using these building blocks, we create devices based on dimerised chains to perform an analogue quantum simulation of the Su-Schrieffer-Hegger model, a topological quantum system that classical computers cannot solve effectively. As a preliminary result, we demonstrate the formation of 25 nm-long electronic bands in chains of 5 and 7 sites. These results demonstrate the viability of using silicon as a base for the realisation of Fermi-Hubbard analogue quantum simulators inside a low-temperature STM, with the tip as a tuneable local probe.
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(2023)ThesisSilicon 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.
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(2023)ThesisQuantum networks are the quantum analogue of the modern internet, offering the ability to employ more secure protocols than modern communication. At the base of these quantum networks are so-called qubits: physical entities that can be in a simultaneous superposition of two states. For quantum networks, qubits should contain states that can be optically accessed and can store quantum properties for a long time. Nowadays, a number of qubit platforms exist, with spin qubits in Si offering long coherence times due to the low number of nuclear spins in the host crystal. Si is a widely used semiconductor in modern electronic devices and the maturated micro- and nanofabrication techniques can be exploited to miniaturise quantum devices. Additionally, Si and SiO2 form an attractive photonic material system because they provide a large contrast in the refractive index, which is critical for achieving efficient optical confinement. To minimise the photon losses and leverage on the well-established telecommunication networks, the photons excited from the spin states should emit within the telecommunication C-band. In this sense, Er3+:Si is an attractive system because Er3+ ions exhibit a spin transition that can be accessed by photons with frequencies within the telecommunication C-band. Moreover, the optical transition of Er3+ takes place within an inner shell, meaning that the outer shells electrically shield this transition, resulting in narrow and stable optical transitions within the telecommunication C-band. The upper bounds on the coherence times of the optical and spin transitions of Er3+:Si are, thus far, still unknown. In this thesis, these optical and spin properties of Er3+ ions in Si are investigated with the aim to create an interface between a spin qubit and a flying qubit. Here, the optical measurements include the extraction of inhomogeneous and homogeneous broadening of Er3+:Si over various samples, observing linewidths down to less than 100 MHz and 500 kHz, respectively. The low Er3+ density in natural Si samples showed characteristics of long-lived electron spin states for two sites. The electron spin coherence time of an Er3+ site in a nuclear-spin-free Si crystal was measured to be 0.5 ms by employing a Hahn echo sequence, and this was further extended up to 9 ms using a Carr–Purcell Meiboom–Gill sequence. These optical and spin properties establish that Er3+:Si exhibits fundamentally promising properties for quantum networks.