| Summary: | <p>Superconducting circuits have been demonstrated at the ~50 qubit scale, with gate fidelities approaching the thresholds required for error correction using surface codes. It is generally considered that maintaining and improving this performance while scaling to >100 qubits will require 3D integration of control wiring, and techniques to prevent or mitigate the effects of unwanted electromagnetic modes which emerge in the circuit environment at larger scales. At the present time, no single design has emerged as the clear scaling "winner'', and many different modalities are being explored. In this thesis, we describe one potential scaling architecture for superconducting circuits, which is based on a unit cell containing a set of controllable, measurable qubits. In analogy to the unit cell of a crystal, this unit cell can be tiled, in this case to form 2D arrays of individually controllable and measurable qubits. This is enabled by three key technologies: 3D integrated off-chip control wiring, reverse-side readout resonators, and inductive shunting of the device enclosure with micromachined pillars. To validate this approach, we design and build a demonstration device featuring four transmon qubits and containing within it a single unit cell of the architecture.</p>
<p>The device construction involves the fabrication of reverse-side circuits and the use of novel CNC drilling of the circuit substrate. We measure coherence times and single-qubit gate fidelities comparable with the state-of-the-art for transmon qubits (avg. T1 = 149 µs, avg. echoed pure dephasing T2 = 189 µs, avg. F > 99.98%), establishing that the design is compatible with high coherence. An important feature of the design is that the infinite construct formed by tiling the plane with the unit cell possesses a cutoff frequency, below which the circuit environment cannot sustain any electromagnetic modes. We develop models to predict this cutoff frequency, and predict that environment mediated crosstalk between qubits inside this structure decays exponentially with spatial separation. This crosstalk can be viewed as being mediated by evanescent waveguide modes, or by bound states formed by the interaction of qubits with the environmental modes around the cutoff. Both models provide quantitative predictions for the rate of crosstalk decay, which are shown to be in good agreement. We propose this architecture as a promising candidate for building 2D arrays of hundreds or thousands of nearest neighbour coupled superconducting qubits, which could provide a testbed of quantum error correction schemes such as the surface code.</p>
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