Protected Hybrid Superconducting Qubit in an Array of Gate-Tunable Josephson Interferometers

We propose a protected qubit based on a modular array of superconducting islands connected by semiconductor Josephson interferometers. The individual interferometers realize effective cos⁡2ϕ elements that exchange “pairs of Cooper pairs” between the superconducting islands when gate tuned into balance and frustrated by a half-flux quantum. If a large capacitor shunts the ends of the array, the circuit forms a protected qubit because its degenerate ground states are robust to offset-charge and magnetic field fluctuations for a sizable window around zero offset charge and half-flux quantum. This protection window broadens upon increasing the number of interferometers if the individual elements are balanced. We use an effective spin model to describe the system and show that a quantum phase transition sets the critical flux value at which protection is destroyed.