Dissertation Committee Chair: Vladimir Manucharyan
Committee:
Professor Mohammad Hafezi
Professor Alicia Kollar
Professor Christopher Lobb
Professor Jay Deep Sau
Abstract: Chains of Josephson junctions are known to produce some of the largest kinetic per-unit-length inductance, which can exceed the conventional geometric one by about 10^4. However, the maximum total inductance is still limited by the stray capacitance of the chain, which results in parasitic self-resonances. This stray capacitance is unnecessarily large in most circuits due to the high dielectric constant of silicon or sapphire substrates used. Here, we explore a regime of ultra-high impedance superconducting circuits by introducing the technique of releasing the Josephson chain off the substrate. The ultra-high impedance regime, defined as an impedance which exceeds 4 times the resistance quantum (4Rq ~ 25.8 kOhms), is realized in this work by combining a maximal per-unit-length inductance with a minimal stray capacitance and demonstrates the highest impedance electromagnetic structures available today. We begin with suspended “telegraph" transmission lines, composed of 30,000+ junctions and show that the wave impedance can exceed 5 times Rq (33 kOhms) while the line still maintains a negligible DC resistance. To quantify the effects of parasitic chain modes in ultra-high impedance circuits, we use high-inductance fluxonium qubits. We show that chain modes are ultra-strongly coupled to the qubit but can be moved to higher frequency with the Josephson chain releasing technique. Finally, we create a superconducting quasicharge qubit (blochnium), dual of transmon, whose impedance reaches over 30 times Rq (200 kOhms) with no evidence of parasitic modes below 10 GHz. This qubit completes the periodic table of superconducting atoms and demonstrates the dual nature of a small Josephson junction in ultra-high impedance circuits, which we probe in a DC experiment in the final chapter.
Location: PSC 1136