Blochnium: A flux-tunable qubit with flux-insensitive coherence times

We introduce a flux-insensitive superconducting artificial atom nicknamed ``blochnium", which is dual to a transmon. A blochnium circuit consists of a small-area Josephson junction shunted by a hyperinductance -- a micro-Henry range linear inductance whose impedance reaches above 160 kΩ. Using innovative circuit design and fabrication tricks, we significantly reduced the parasitic capacitance previously associated with such large inductances. The flux dispersion of the qubit transition is reduced to about 100 MHz while transitions to non-computational states can be tuned in the usual few-GHz range. Such a unique spectrum eliminates flux-noise induced dephasing during flux-controlled logical gates on coupled blochnium qubits, which in theory allows for gate fidelities better than 0.999.