The high-coherence fluxonium qubit

We report superconducting fluxonium qubits with coherence times largely limited by energy relaxation and reproducibly satisfying T2 > 100 microseconds (T2 > 300 microseconds in one device). Moreover, given the state of the art values of the surface loss tangent and the 1/f flux noise amplitude, coherence can be further improved beyond 1 millisecond. Our results violate a common viewpoint that the number of Josephson junctions in a superconducting circuit — over 100 here — must be minimized for best qubit coherence. We outline how the unique to fluxonium combination of long coherence time and large anharmonicity can benefit both gate-based and adiabatic quantum computing.