  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.
