Quarton qubits with stacked Josephson junctions

Superconducting quantum circuits rely on compact, low-loss inductive elements, strong anharmonicity, and robust flux biasing to realize coherent and controllable quantum bits. This thesis investigates how vertically stacked Josephson junction arrays can be employed to meet these requirements and enable compact flux qubits operated in the quarton regime.

In the first part, stacked Al/AlOx/Al Josephson junctions are established as compact linear inductors. Their three-dimensional geometry reduces parasitic capacitance to ground while allowing multiple junctions to be integrated within a small footprint. A numerical modeling approach is used to quantify stray capacitances, Coulomb screening, and collective modes of extended stacked junction arrays. Cryogenic transport measurements confirm strongly underdamped junction dynamics with uniform junction characteristics, demonstrating their suitability as low-loss inductive elements in superconducting circuits.

In the second part, these inductors are integrated into flux qubits operated in the quarton regime. In this regime, the qubit potential becomes nearly quartic around half-flux bias, resulting in positive anharmonicity and first-order insensitivity to flux noise at the operating point. ... mehrSpectroscopy reveals qubit transition frequencies in the 3–5 GHz range and anharmonicities of 0.5–1.5 GHz, consistent with the expected quartic potential landscape.

To distinguish intrinsic from externally induced decoherence, a persistent-current bias scheme is implemented, enabling stable and low-noise flux biasing over time scales of up to one week. A direct comparison between conventionally biased and persistently biased devices rules out external flux noise as the dominant coherence-limiting mechanism.
High-resolution spectroscopy identifies avoided level crossings caused by microscopic two-level systems (TLS) in the amorphous tunnel barriers of the stacked junctions. A systematic cumulative analysis of splitting statistics across multiple devices shows that the spectral TLS density scales with the total tunnel-barrier area, while the array length determines the effective qubit–TLS coupling strength. The measured relaxation times follow scaling relations consistent with Fermi’s golden rule for a bath of weakly coupled TLS, demonstrating that TLS in the stacked junction arrays constitute the dominant intrinsic decoherence channel in quarton qubits.

These results establish stacked Josephson junction arrays as compact superinductive elements and provide an understanding of coherence limitations in superconducting quantum circuits based on Josephson junction arrays, offering guidance for future device optimization.