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Design of tunable couplers and investigation of loss mechanisms in superconducting 2D and 3D systems
Materise, Nicholas R.
Materise, Nicholas R.
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2023
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Superconducting qubits are among the leading physical qubit candidates, with coherence times exceeding 100 microseconds and gate times on the order of tens of nanoseconds. At the hardware level, tunable coupling between qubits has enabled fast, high fidelity two qubit gates, the limiting factor for quantum algorithm gate depth. Tunable couplers have played a significant role in scaling these systems and were instrumental in the first quantum advantage demonstration in certifiable random number generation. Parametric operations, such as beam splitter and two-mode squeezing, are activated by oscillating fields applied to a tunable coupler that are resonant with these red and blue sidebands, respectively. Theoretical modeling tools exist for planar geometries and 3D geometries that use capacitive coupling, but ones for galvanic coupling in 3D have not been realized.
This thesis will discuss the design of two novel tunable couplers, one in the planar domain and another in 3D that uses galvanic coupling. In the planar design, a III-V semiconductor heterostructure acts as a tunable capacitor when biased with a negative gate voltage, parting the sea of electrons to modify the geometry of the capacitor. The 3D tunable coupler uses a dc superconducting quantum interference device shunting a 3D cavity and driven at the sum or difference frequencies of cavities to induce beam splitter or two-mode squeezing operations. I will discuss this 3D galvanic coupler and the analysis method developed to estimate beam splitter, two-mode squeezing, and single-mode squeezing rates.
The novel materials comprising the 2DEG coupler spurred experiments to estimate its dielectric losses. These experiments led to the design of cavity-based loss metrology systems, with applications in a variety of materials of interest to the superconducting qubit field, namely bulk dielectric loss in indium phosphide, silicon, and sapphire substrates. I will discuss these cavity loss experiments and 2D resonator loss measurements focused on understanding loss mechanisms in 2D qubits. The materials in the 2D studies include niobium and its oxides and hydrides, tantalum, titanium nitride, and silicon. These studies highlight the importance of designing targeted A/B experiments and collecting sufficient loss statistics with tens of resonators per device variation.
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