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Nonlinear superconducting circuit elements for quantum technologies : from design to experiment Elkholy, Karim


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 inductance of 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 the typical single-wire device. Further investigation is needed to understand this, and to investigate whether or not the tunability can be increased beyond 28%.

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