We investigate die-level and wafer-scale uniformity of Dolan-bridge and bridgeless Manhattan-style Josephson junctions, using multiple substrates with and without through-silicon vias (TSVs). Dolan junctions fabricated on planar substrates have the highest yield and lowest room-temperature conductance spread, equivalent to ∼ 100 M H z in transmon frequency. In TSV-integrated substrates, Dolan junctions suffer most in both yield and disorder, making Manhattan junctions preferable. Manhattan junctions show pronounced conductance decrease from wafer center to edge, which we qualitatively capture using a geometric model of spatially-dependent resist shadowing during junction electrode evaporation. Analysis of actual junction overlap areas using scanning electron micrographs supports the model, and further points to a remnant spatial dependence possibly due to contact resistance.

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We present the use of a set of airbridges to trim the frequency of microwave coplanar-waveguide (CPW) resonators post-fabrication. This method is compatible with the fabrication steps of conventional CPW airbridges and crossovers and increases device yield by allowing compensation of design and fabrication uncertainty with 100 MHz range and 10 MHz resolution. We showcase two applications in circuit QED. The first is the elimination of frequency collisions between resonators intended to readout different transmons by frequency-division multiplexing. The second is frequency matching of readout and Purcell-filter resonator pairs. Combining this matching with transmon frequency trimming by laser annealing reliably achieves fast and high-fidelity readout across 17-transmon quantum processors.

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Quantum computers promise to speedup certain problems that conventional computers take too long to solve. These problems include nitrogen fixation, quantum chemistry and prime factorization. One promising platform for the implementation of a practical quantum computer are superconducting qubits in combination with circuit quantum electrodynamics (CQED). However, preventing the large scale application of quantum computers is noise and decoherence, limiting the size and depth of a quantum algorithm. Particularly flux noise plagues tunable qubits, limiting their flexibility and fidelity. One of the most used and advanced qubits is the transmon, a LC oscillator with a capacitor in parallel with a non-linear inductive element called a Josephson junction. Conventionally, the Josephson junction is formed with an Al-AlO-Al tunnel barrier. Contrastingly, here we use a InAs nanowire covered with a thin layer of Al forming a S-N-S Josephson junction. Crucially, this junction is magnetic field compatible, allowing us to do experiments with cQED in a magnetic field. Additionally this junction is voltage-tunable, opening the path towards lower distortion voltage gates. This thesis focusses on measuring the flux noise in a magnetic field using the nanowire Josephson junction. To that end, the chapters address the necessary conditions to achieve this goal....@en

Quantum hardware based on circuit quantum electrodynamics makes extensive use of airbridges to suppress unwanted modes of wave propagation in coplanar-waveguide transmission lines. Airbridges also provide an interconnect enabling transmission lines to cross. Traditional airbridge fabrication produces a curved profile by reflowing resist at elevated temperature prior to metallization. The elevated temperature can affect the coupling energy and even yield of pre-fabricated Josephson elements of superconducting qubits, tunable couplers, and resonators. We employ grayscale lithography to enable reflow and thereby reduce the peak temperature of our airbridge fabrication process from 200 to 150 °C and link this change to a substantial increase in the physical yield of transmon qubits with Josephson elements realized using Al-contacted InAs nanowires.

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Ultrafast scanning electron microscopy images carrier dynamics and carrier induced surface voltages using a laser pump electron probe scheme, potentially surpassing all-optical techniques in probe resolution and surface sensitivity. Current implementations have left a four order of magnitude gap between optical pump and electron probe resolution, which particularly hampers spatial resolution in the investigation of carrier induced local surface photovoltages. Here, we present a system capable of focusing the laser using an inverted optical microscope built into an ultrafast scanning electron microscopy setup to enable high numerical aperture pulsed optical excitation in conjunction with ultrafast electron beam probing. We demonstrate an order of magnitude improvement in optical pump resolution, bringing this to sub-micrometer length scales. We further show that temporal laser pump resolution can be maintained inside the scanning electron microscope by pre-compensating dispersion induced by the components required to bring the beam into the vacuum chamber and to a tight focus. We illustrate our approach using molybdenum disulfide, a two-dimensional transition metal dichalcogenide, where we measure ultrafast carrier relaxation rates and induced negative surface potentials between different flakes selected with the scanning electron microscope as well as on defined positions within a single flake.

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We present an experimental study of flux- and gate-tunable nanowire transmons with state-of-the-art relaxation time allowing quantitative extraction of flux and charge noise coupling to the Josephson energy. We evidence coherence sweet spots for charge, tuned by voltage on a proximal side gate, where first order sensitivity to switching two-level systems and background 1/f noise is minimized. Next, we investigate the evolution of a nanowire transmon in a parallel magnetic field up to 70 mT, the upper bound set by the closing of the induced gap. Several features observed in the field dependence of qubit energy relaxation and dephasing times are not fully understood. Using nanowires with a thinner, partially covering Al shell will enable operation of these circuits up to 0.5 T, a regime relevant for topological quantum computation and other applications.

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