  The non-dissipative non-linearity of a Josephson junction converts
macroscopic superconducting circuits into artificial atoms, enabling some of
the best controlled quantum bits (qubits) today. Three fundamental types of
superconducting qubits are known, each reflecting a distinct behavior of
quantum fluctuations in a Cooper pair condensate: single charge tunneling
(charge qubit), single flux tunneling (flux qubit), and phase oscillations
(phase qubit). Yet, the dual nature of charge and flux suggests that circuit
atoms must come in pairs. Here we introduce the missing one, named "blochnium".
It exploits a coherent insulating response of a single Josephson junction that
emerges from the extension of phase fluctuations beyond the $2\pi$-interval.
Evidence for such effect was found in an out-of-equilibrium dc-transport
through junctions connected to high-impedance leads, although a full consensus
is absent to date. We shunt a weak junction with an exceptionally high-value
inductance -- the key technological innovation behind our experiment -- and
measure the rf-excitation spectrum as a function of external magnetic flux
through the resulting loop. The junction's insulating character manifests by
the vanishing flux-sensitivity of the qubit transition between the ground and
the first excited states, which nevertheless rapidly recovers for transitions
to higher energy states. The spectrum agrees with a duality mapping of
blochnium onto transmon, which replaces the external flux by the offset charge
and introduces a new collective quasicharge variable in place of the
superconducting phase. Our result unlocks the door to an unexplored regime of
macroscopic quantum dynamics in ultrahigh-impedance circuits, which may have
applications to quantum computing and quantum metrology of direct current.
