  The superconducting fluxonium circuit is an artificial atom with a strongly
anharmonic spectrum: when biased at a half flux quantum, the lowest qubit
transition is an order of magnitude smaller in frequency than those to higher
levels. Similar to conventional atomic systems, such a frequency separation
between the computational and noncomputational subspaces allows independent
optimizations of the qubit coherence and two-qubit interactions. Here we
describe a controlled-Z gate for two fluxoniums connected either capacitively
or inductively, with qubit transitions fixed near 500 MHz. The gate is
activated by a microwave drive at a resonance involving the second excited
state. We estimate intrinsic gate fidelities over 99.9% with gate times below
100 ns.

Quantum Physics;Mesoscale and Nanoscale Physics