Fabrication of air-bridges for qubit design

Examensarbete för masterexamen
Master Thesis
Abuwasib, Mohammad
Superconducting quantum electrical circuit based on Co-planar waveguide technology is considered one of the most promising candidates for practical realization of a quantum computer in future. For purpose of designing a multi-qubit quantum processor which is the heart of a quantum computer, it is required to couple qubits to a large number of superconducting transmission lines and resonators. Placing all these transmission lines and resonators in a practical design unavoidably creates bending and discontinuities in the ground planes which have higher order parasitic modes that can be excited. These undesired parasitic modes are detrimental for quantum measurement, for example during readout of a qubit. Unwanted modes can easily interfere with a resonator frequency when coupling to the qubit. These parasitic modes can propagate throughout CPW structure and make unequal ground potential at the corresponding ground planes. The easiest way to suppress these parasitic modes is to connect two unequal ground potential with superconducting air-bridges in order to bring them under equal potential in superconducting state. However, connecting airbridges on top of CPW adds shunt capacitor which may cause undesired reflection in the CPW. In this thesis work, we designed superconducting air-bridges on top of superconducting CPW transmission line which is a subsystem of large quantum electrical circuit. Our focus was to reduce added shunt capacitance by choosing the right height of these air-bridges. To keep the reflection coming from the air-bridge at minimum level, an airbridge height of 8.5μm~15μm was chosen. The performance was verified by EM simulation and these heights were selected for device fabrication. We developed a new fabrication method for processing 8.5μm~15μm high aluminum air-bridges for quantum electrical circuits. The method is compatible with the quantum circuit and easily reproducible. Finally, input reflection measurement (S11) at cryogenic temperature was performed for 8.5μm high, 300μm long aluminum air-bridges with aluminum CPW transmission line. Negligible amount of input reflection (S11) was observed from these measurements which is consistent with the result of EM simulation.
Nanovetenskap och nanoteknik , Supraledning , Lågtemperaturfysik , Nanoscience & Nanotechnology , Superconductivity , Low temperature physics
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