The work presented in this thesis focuses on two critical components required for a scheme of scalable quantum computer architecture: high-fidelity two-qubit gate operations, and the demonstration of position-dependent quantum logic operations. The first part of this thesis is centered on the assessment of two-qubit gate schemes. Initially, two Mølmer-Sørensen type gates are examined. These gates employ a set of tailored quantum control methods, exhibiting enhanced robustness to the effects of noise compared to the standard Mølmer-Sørensen gate interaction, and further rendering their implementation on the magnetic field sensitive Zeeman-state qubit possible. The advantages arising from their quantum control methods are presented, along with their limitations. Subsequently, the three-level dressed-state Mølmer-Sørensen gate is examined, which combines the advantages of the Mølmer-Sørensen gate interaction with the dephasing-free dressed-state qubit subspace. Methods of efficiently calibrating this two-qubit gate are presented. A two-qubit gate fidelity of F=0.951(11) is measured, which is found to be limited by the effects of voltage noise present in our setup. The second part of this thesis is centered on the demonstration of position-dependent quantum logic operations. Initially, it is demonstrated that a clock-state qubit can be transported through a static magnetic field gradient. Furthermore, a mapping sequence that can transfer a qubit state from the dressed-state computational qubit basis to the shuttling clock-qubit basis is presented. A series of ten characterisation experiments are then detailed, which both demonstrate and further characterise the effects of shuttling a dressed-state qubit through a static magnetic field gradient. It is observed that the shuttling operations induce a phase offset on the dressed-state qubit when shuttled through the gradient. Finally, two modifications to the CNOT gate quantum circuit are presented, which account for the phase offsets induced by the shuttling operations on the computational qubit basis. These operations pave the way towards the demonstration of position-dependent quantum logic operations on a modular quantum computing architecture.
The work presented in this thesis focuses on two critical components required for a scheme of scalable quantum computer architecture: high-fidelity two-qubit gate operations, and the demonstration of position-dependent quantum logic operations. The first part of this thesis is centered on the assessment of two-qubit gate schemes. Initially, two Mølmer-Sørensen type gates are examined. These gates employ a set of tailored quantum control methods, exhibiting enhanced robustness to the effects of noise compared to the standard Mølmer-Sørensen gate interaction, and further rendering their implementation on the magnetic field sensitive Zeeman-state qubit possible. The advantages arising from their quantum control methods are presented, along with their limitations. Subsequently, the three-level dressed-state Mølmer-Sørensen gate is examined, which combines the advantages of the Mølmer-Sørensen gate interaction with the dephasing-free dressed-state qubit subspace. Methods of efficiently calibrating this two-qubit gate are presented. A two-qubit gate fidelity of F=0.951(11) is measured, which is found to be limited by the effects of voltage noise present in our setup. The second part of this thesis is centered on the demonstration of position-dependent quantum logic operations. Initially, it is demonstrated that a clock-state qubit can be transported through a static magnetic field gradient. Furthermore, a mapping sequence that can transfer a qubit state from the dressed-state computational qubit basis to the shuttling clock-qubit basis is presented. A series of ten characterisation experiments are then detailed, which both demonstrate and further characterise the effects of shuttling a dressed-state qubit through a static magnetic field gradient. It is observed that the shuttling operations induce a phase offset on the dressed-state qubit when shuttled through the gradient. Finally, two modifications to the CNOT gate quantum circuit are presented, which account for the phase offsets induced by the shuttling operations on the computational qubit basis. These operations pave the way towards the demonstration of position-dependent quantum logic operations on a modular quantum computing architecture.
This thesis describes experimental work on implementing single and two qubit gates in ¹7¹Yb+ ions using methods suitable for a large scale quantum computer. By combining a magnetic field gradient with microwave and radiofrequency radiation, the spin and motional states of the ions are coupled which allows multi-qubit operations to be performed, as well as providing individual addressing of ions in frequency space. A dressed state qubit is used which exhibits an increase of over two orders of magnitude in the coherence time for a qubit that it is sensitive to the magnetic field gradient. Using this system, single qubit gates are characterised using the technique of randomised benchmarking, resulting in a measured average error per gate of 9(3)x10¯4. A new type of two qubit gate is experimentally demonstrated, which in comparison to a standard two qubit gate shows significantly increased resilience to two major sources of gate infidelity: heating of the motional mode of the ions during gate operations, and incorrectly set gate field frequencies. These types of errors are expected to become increasingly important with the move towards quantum processors with large numbers of qubits. Using this same technique, a two qubit gate is also demonstrated at a higher initial temperature with a significantly improved fidelity compared to standard methods. These gate techniques are used to demonstrate work towards implementing positiondependent quantum logic, a method which could remove the correlation between the number of ions and the number of gate fields required in a large scale quantum computing architecture. A method to move the dressed state qubit through a magnetic field gradient while preserving quantum information is demonstrated, as well as a method to optimise the phase of a two qubit gate of unknown phase in order to implement a CNOT logic gate. This provides a path forwards to demonstrating a CNOT gate using position-dependent quantum logic.
This thesis describes the development of technology and experimental techniques for scalable trapped-ion quantum computing. An architecture for scalable quantum computing is presented in which 171Yb+ ions are trapped in linear R.F. Paul traps and their hyper-fine states are used to form qubits. Quantum logical operations can be carried out by applying R.F. and microwave fields. This thesis presents significant improvements in the experimental setup used for quantum computing, as well as new theoretical developments and experimental demonstrations of new high- fidelity quantum control methods. A new procedure for carrying out high- delity two qubit gates using static magnetic eld gradients and resonant microwave elds is presented. It is shown that this has the potential to enable signi cantly improved delities compared to current methods. Improved methods for measurement and statistical analysis in trapped-ion experiments are also shown. These should enable more accurate measurement of two-qubit gate delities, which becomes increasingly important as delities increase. A new system for generating arbitrary R.F. and microwave waveforms is presented, including a bespoke software control system allowing the user to de ne custom pulse sequences, which represents a signi cant improvement in the scalability of the quantum control system. Experimental developments towards a system for carrying out on-chip trapped-ion quantum logic in a static magnetic eld gradient are also presented. A new technique for generating multi-level control methods is introduced, based on reducing a multi-level system to an e ective two-level system. High- delity quantum control methods generated using this technique, which form a will form a key part of the scalable quantum computing architecture, are implemented experimentally.
Since the mid-20th century it has been understood that a general-purpose quan- tum computer would be able to efficiently solve problems that will forever be out-of-reach for conventional computers. Since then, many quantum algorithms have been developed with applications in a wide range of domains including cryptography, simulations, machine learning and data analysis. While this has resulted in substantial attention being paid to the development of quantum com- puters, the best architectures to use in their fabrication is not yet clear. Semiconductor quantum dot devices are a particularly promising candidate for use in quantum computing architectures, as it is anticipated that once the funda- mental building blocks are implemented, they might be massively scalable using the existing lithography techniques of the semiconductor industry. So far, how- ever, it is not yet clear how best to implement the high-fidelity gates required for general-purpose quantum computation. In this thesis, we present and characterise novel theoretical proposals for fast, simple and high-fidelity two-qubit gates using magnetic (exchange) coupling for specific semiconductor quantum dot qubits; namely, the singlet-triplet and resonant-exchange qubits. These two-qubit operations are simple enough that it is feasible for them to be implemented in experiments of the near future. Success- ful implementations would significantly extend the experimentally demonstrable frontier of semi-conductor quantum dot devices as relevant to their use in uni- versal quantum computing architectures. We also develop simple parameter estimation schemes by which it is possible to substantially mitigate the dominant sources of error for our proposed gates; namely, low-frequency charge and magnetic noise. We develop the techniques in the context of pseudo-static magnetic field gradient fluctuations in singlet- triplet qubits, and demonstrate that these techniques lead to a several orders of magnitude improvement in single-qubit coherence times. With minimal effort this could be ported to other qubit architectures.
In the current landscape of quantum technologies, great efforts are being made to overcome the technical challenges of realising robust and scalable devices for use in a wide variety of potentially impactful applications. This thesis therefore describes the design considerations and assembly of a purpose-built vacuum system for the realisation of high-fidelity quantum logic operations on a microfabricated surface ion trap device. The device consists of an X-junction structure with embedded current-carrying wires, which will allow for the demonstration of scalable, laser-free, magnetic field gradient-based quantum logic. Initial characterisation experiments and observations pertaining to this novel microchip design are presented, as well as demonstrations of coherent quantum operations using the 171Yb+ qubit both with and without the current-carrying wire generated magnetic field gradient. In addition, this work also shows how this static magnetic field gradient design for scalable quantum computing may be leveraged for use in metrology. By utilising the inherent gradient-induced coupling of the trapped ion qubit’s transition frequency to it’s position in this architecture, the ion becomes extremely sensitive to electric field perturbations, enabling the implementation of many well-established quantum electric and magnetic field sensing techniques. Furthermore, the nature of this novel sensing scheme permits high sensitivities within the unique DC − kHz bandwidth, which many current quantum electrometers are unable to reach. Measurements of DC and AC electric fields yield unprecedented sensitivities for the sensing of phase coherent electric fields in a linear Paul trap. In addition, an implementation of a common magnetometry technique to measure electric field noise is also exhibited. Finally, it is shown that a purpose built system for the implementation of this scheme has the potential to improve sensitivities by up to six orders of magnitude, enabling the possibility of utilising this scheme for a wide variety of applications'
Quantum physics describes the universe on a microscopic scale. Even though this scale is not accessible without complex experimental setups, quantum effects are respon- sible for the properties of matter and in some instances we have learned to exploit them technologically. A modern computer constituted of transistors passively utilizes the quantum nature of charge carriers in the band structure of semiconductors to encode and process information. Although such a classical computer is a formidable tool already, what could be achieved with active control over the quantum states in the computer is an intriguing question. This is the idea of the quantum computer, which offers an entirely new paradigm of computing by encoding information in quantum binary digits (qubits). Indeed, for several applications it has been shown that a quantum computer is superior to its classical counterpart, even though its realization remains elusive up to date. One candidate to host qubits are gate-defined semiconductor quantum dots, confin- ing single or few charge carriers with spin. The spin of a single confined electron is a quantum two-level system which can be manipulated and measured, allowing the storage and processing of quantum information. This thesis can be viewed as part of the endeavor to give birth to quantum computers based on semiconductor spin qubits. It primarily focuses on the challenge to enable scalable architectures of well-characterized qubits. In the first part, spin shuttling is investigated. This denotes the coherent transport of a single spin between spatially separate nodes of a quantum processor, driven by a time-dependent electric potential. The object of study is a minimal version of a bucket- brigade-like spin shuttling between two quantum dots. The probability of maintaining the spin state is identified as figure of merit and the effect of spin-orbit interaction and the Zeeman effect in an inhomogeneous magnetic field is explored with analytical and numerical methods. Special attention is paid to the repercussions of near-degenerate valleys. The second part of the thesis addresses the challenge to probe the microscopic properties of a quantum dot array, which is a crucial first step before any quantum operation can be performed. The transmission of a microwave resonator coupled to a triple quantum dot occupied by two electrons is investigated, and a scheme to infer the exchange interaction between electrons in adjacent quantum dots is proposed. By means of analytical derivations it is shown that the transmission profile of the resonator directly reveals the value of the exchange coupling strength between two electrons. This holds even if magnetic gradients or a valley splitting comparable to the inter-dot tunnel coupling are included. Subsequently, quantum transport measurements in the Pauli blockade regime are modelled in the presence of a microwave resonator, bringing together two powerful techniques to probe quantum dots. Based on generalized input-output theory, a theoretical framework is derived to investigate how a double quantum dot in a transport setup interacts with a coupled microwave resonator while the current through the double qauntum dot is rectified by Pauli blockade. It is shown that the output field of the resonator can be used to infer the leakage current and thus obtain insight into the blockade mechanisms and to estimate a large number of parameters of the quantum dots. Furthermore, the back-action of the resonator photons on the steady state leakage current is described and quantified. In the context of Pauli blockade, the spin and valley physics of bilayer graphene quantum dots in the few-electron regime is touched upon, describing a situation where spin blockade is observed despite the presence of the valleys. In the final part of the thesis, the attention is shifted towards the readout of the spin qubits. A new paradigm of dispersive readout is devised, which is performed while the qubit dephases, implementing a measurement in the x-basis. Laplace transforming the time dependent cavity response allows a separation of contributions from multiple qubits coupled to the same resonator. Thus, they can be read out simultaneously in a single shot. Modelling silicon spin qubits, as an example, results in in a readout time that compares favourably to conventional dispersive readout with comparable fidelity, even for a single qubit. With an auxiliary qubit the readout scheme can be extended to a quantum non-demolition measurement.
This thesis explores novel strategies for advancing superconducting quantum computing through a series of experimental investigations and device innovations. Grounded in the development of circuit quantum electrodynamics, the work addresses the challenges posed by the limitations of coherent control, coherence time and scaling of superconducting qubits. Focusing on transmon qubits, the thesis first reviews the foundational principles of circuit quantum electrodynamics and the critical role of the transmon in achieving robust qubit performance. Three experimental studies form the core of this work. The first study demonstrates a universal quantum gate set for strongly coupled transmons. By exploiting the residual ZZ interaction, the work introduces a novel two-axis gate protocol for single-qubit operations and implements both CZ and CNOT gates with high fidelities, highlighting the potential for fast, low-error quantum operations in the strong coupling regime. The second study investigates a dual-rail qubit architecture using parametrically coupled transmons, showcasing a hardware-efficient approach to quantum error correction. This experiment converts single-photon loss errors into detectable erasures and leverages mid-circuit detection to enhance qubit coherence, thereby advancing fault-tolerant quantum computing schemes in the NISQ era. The final study focuses on the development of merged-element transmons (MET) and their FinMET variants. By integrating the Josephson junction and shunt capacitor into a single trilayer device, these innovations achieve a dramatic reduction in device footprint while addressing loss channels through advanced materials engineering, ultimately paving the way for scalable, high-coherence quantum circuits. Collectively, these contributions demonstrate significant progress in quantum gate design, error mitigation, and qubit architecture, offering promising routes toward the realization of largescale, fault-tolerant quantum computers