The non-dissipative nonlinearity of Josephson junctions(1)converts macroscopic superconducting circuits into artificial atoms(2), enabling some of the best-controlled qubits today(3,4). Three fundamental types of superconducting qubit are known(5), each reflecting a distinct behaviour of quantum fluctuations in a Cooper pair condensate: single-charge tunnelling (charge qubit(6,7)), single-flux tunnelling (flux qubit(8)) and phase oscillations (phase qubit(9)or transmon(10)). Yet, the dual nature of charge and flux suggests that circuit atoms must come in pairs. Here we introduce the missing superconducting qubit, 'blochnium', which exploits a coherent insulating response of a single Josephson junction that emerges from the extension of phase fluctuations beyond 2 pi (refs.(11-14)). Evidence for such an effect has been found in out-of-equilibrium direct-current transport through junctions connected to high-impedance leads(15-19), although a full consensus on the existence of extended phase fluctuations is so far absent(20-22). We shunt a weak junction with an extremely high inductance-the key technological innovation in our experiment-and measure the radiofrequency excitation spectrum as a function of external magnetic flux through the resulting loop. The insulating character of the junction is manifested by the vanishing flux sensitivity of the qubit transition between the ground state and the first excited state, which recovers rapidly for transitions to higher-energy states. The spectrum agrees with a duality mapping of blochnium onto a transmon, which replaces the external flux by the offset charge and introduces a new collective quasicharge variable instead of the superconducting phase(23,24). Our findings may motivate the exploration of macroscopic quantum dynamics in ultrahigh-impedance circuits, with potential applications in quantum computing and metrology. A fundamental superconducting qubit is introduced: 'blochnium' is dual to the transmon, relies on a circuit element called hyperinductance, and its fundamental physical variable is the quasicharge of the Josephson junction.