A superconducting qubit with noise-insensitive plasmon levels and decay-protected fluxon states

Abstract

The inductively shunted transmon (IST) is a superconducting qubit with exponentially suppressed fluxon transitions and a plasmon spectrum approximating that of the transmon. It shares many characteristics with the transmon but offers charge offset insensitivity for all levels and precise flux tunability with quadratic flux noise suppression. In this work we propose and realize IST qubits deep in the transmon limit where the large geometric inductance acts as a mere perturbation. With a flux dispersion of only 5.1 MHz we reach the 'sweet-spot everywhere' regime of a SQUID device with a stable coherence time over a full flux quantum. Close to the flux degeneracy point the device reveals tunneling physics between the two quasi-degenerate ground states with typical observed lifetimes on the order of minutes. In the future, this qubit regime could be used to avoid leakage to unconfined transmon states in high-power read-out or driven-dissipative bosonic qubit realizations. Moreover, the combination of well controllable plasmon transitions together with stable fluxon states in a single device might offer a way forward towards improved qubit encoding schemes.

13 pages, 8 figures

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• Magnetic Field Tolerant Superconducting Spiral Resonators for Circuit QED
• Sub-Harmonic Control of a Fluxonium Qubit via a Purcell-Protected Flux Line
• Proposal for a Quantum Mechanical Test of Gravity at Millimeter Scale
• Qubit-environment entanglement in time-dependent pure dephasing
• Cross-resonance control of an oscillator with an auxiliary fluxonium qubit
• Viewing protected superconducting qubits through the lens of the cat qubit
• Experimental protocol for qubit-environment entanglement detection
• Protected phase gate for the $0$-$\pi$ qubit using its internal modes