Protected hybrid superconducting qubit in an array of gate-tunable Josephson interferometers

TL;DR

This paper introduces a protected superconducting qubit design using an array of gate-tunable Josephson interferometers, offering robustness against charge and magnetic noise, with potential for scalable quantum computing.

Contribution

It proposes a novel protected qubit architecture based on modular superconducting islands and semiconductor Josephson interferometers, enhancing noise resilience.

Findings

The qubit exhibits a broad protection window around zero charge and half flux quantum.

Increasing the number of interferometers enhances the protection window.

A quantum phase transition determines the critical flux where protection fails.

Abstract

We propose a protected qubit based on a modular array of superconducting islands connected by semiconductor Josephson interferometers. The individual interferometers realize effective $\cos2\phi$ 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 point sets the critical flux value at which protection is destroyed.