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Abstract

To continue achieving ever faster computation speeds, future computer processors may need to increase their operating frequency to achieve clock speeds beyond several GHz. Quantum computing offers an alternate approach by leveraging quantum mechanical superposition to make each clock operation more efficient, allowing the processor to solve certain problems much more efficiently. Current quantum processors operate slower than their classical counterparts, with the fastest quantum operations at microwave frequencies and utilizing superconducting artificial atoms (qubits)— a promising platform for quantum experiments studying light-matter interactions in the strong coupling regime. Increasing qubit frequency to the millimeter-wave range (∼100 GHz) offers a straightforward way to increase quantum computing speed for any qubit design. Crucially, millimeter-wave frequencies also have greatly reduced sensitivity to thermal noise. Whereas microwave qubits require extremely low temperatures (< 50 mK) through isotopic enrichment of 3He and 4He in order to reduce sources of decoherence, millimeter-wave qubits can operate at significantly higher temperatures near 1 K. These temperatures are achievable using simpler methods such as 4He evaporation, which translates to orders of magnitude higher cooling power. This is transformative for scaling up superconducting quantum processors by significantly increasing the number of qubit control channels possible in a single cryostat, enabling direct integration of qubits with superconducting digital processors, and allowing for more energy efficient possibilities for quantum communication between cryostats. In this thesis, we introduce millimeter-wave superconducting devices as a platform for quantum experiments. We develop a robust niobium trilayer Josephson junction with improved quantum coherence properties capable of operating at higher frequencies and temperatures than conventional aluminum junctions. Based on this technology we explore the thermal resilience of qubits with higher and higher frequency, finally demonstrating a 72 GHz millimeter-wave qubit cooled entirely with helium-4.

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