Blochnium: A flux-insensitive qubit with flux-tunable interactions

We introduce a flux-like qubit, nicknamed 'blochnium', whose 0-1 transition frequency is nearly insensitive to the external flux bias and is dual to a transmon. Blochnium is created by shunting a small Josephson junction with a hyperinductance, that is -- a maximal per-unit-length inductance that has minimal stray capacitance and is probably the highest impedance electromagnetic structure available today (L_tot > 1  μH, Z > 200 kΩ). To achieve such inductances, we release the entire circuit from the high dielectric substrate and suspend it almost entirely in vacuum. The qubit’s spectrum reveals a 0-1 flux dispersion on the order of a hundred MHz which becomes exponentially suppressed when decreasing the ratio EL/EC providing protection against 1/f flux noise. However, the higher transitions remain flux-sensitive allowing for multi-qubit operations. We report on recent coherence properties of blochnium with varying etching techniques.