Joseph Salfi

Large-scale quantum computers have the potential to perform calculations that are otherwise impossible, a capability that could power exciting advances in fields such as materials design and optimization. Building large-scale quantum computers with spin qubits is appealing because they have long coherence times and can be fabricated on silicon chips using an industrial process amenable to scaling. State-of-the-art spin qubit systems are still small, having only just reached the 2-qubit and 4-qubit scale, and their performance and scalability are not optimized yet. Connecting large numbers of spin qubits on chips remains a challenge. In this thesis we demonstrate a simplified fabrication process using a single layer of gates to realize hole spin qubits, anticipated to be easier to scale up than conventional approaches, based on quantum dots formed in a germanium quantum wells on silicon substrates. We also devised a novel approach to reduce contact resistance to the quantum well. Using this process we successfully built quantum dots, as evidenced by Coulomb blockade spectroscopy. Future work will demonstrate quantum bits using this process.Optimization of qubits based on quantum devices requires cooling them down below 4 Kelvin and connecting them to microwave control and measurement circuits. Designing a high frequency control and measurement apparatus is challenging since it requires suppression of stray resonances and crosstalk in the setup.Typically each research group designs its own apparatus, or purchases an expensive apparatus that is not possible to customize. In this thesis, we design and test an apparatus for controlling and measuring few-qubit devices using low-frequency and microwave electrical signals, that can be used to optimize qubit devices.Our setup has -40 dB cross-talk with no resonances up to 7 GHz, and has the advantage of being small in size (
View record

Nonlinearity in superconducting devices has proved to be an essential part of quantum computing. It produces the anharmonicity needed for superconducting qubits, gives rise to parametric amplification, and enhances qubit readout. Although nonlinearity has been thoroughly investigated in Josephson Junction-based devices (JJ), the performance of single JJ devices is hampered by higher-order nonlinearities, and JJ arrays that can overcome this are difficult to fabricate. Moreover, JJ devices suffer from small critical currents, which limits the dynamic range of the device. This spurred the interest towards investigating the Nanowire (NW)-based kinetic inductance devices that offer a naturally distributed non-linearity, ease of fabrication and higher critical currents. Although they have been demonstrated as near-ideal parametric amplifiers, they suffer from weak inductance tunability (weaker nonlinearity) than JJ devices which is a key property in some applications. In this thesis, we investigate two different designs of a more tunable NW-based inductor that tackle the challenges found in the state-of-the-art superconducting devices whose tunability was limited to around 28% using the kinetic inductanceof a single wire. They are based on different approaches where we tune either penetration depth or kinetic inductance. The latter proposal is more sophisticated yet more promising, so it is what we proceed our experiment with. The novel device works by sharing current between parallel inductors with different critical currents and inductances, in order to reduce the impact of fluctuations, e.g., thermal noise or vortices, on tunability in the state-of-the-art. We measured a tunability of 1.6% in this device, higher than the measured 0.4% in our realization of thetypical single-wire device. Further investigation is needed to understand this, and to investigate whether or not the tunability can be increased beyond 28%.

View record