Superconducting circuit protected by two-Cooper-pair tunneling

W C Smith,A Kou,U Vool,M H Devoret,X Xiao

doi:10.1038/s41534-019-0231-2

Abstract

We present a protected superconducting qubit based on an effective circuit element that only allows pairs of Cooper pairs to tunnel. These dynamics give rise to a nearly degenerate ground state manifold indexed by the parity of tunneled Cooper pairs. We show that, when the circuit element is shunted by a large capacitance, this manifold can be used as a logical qubit that we expect to be insensitive to multiple relaxation and dephasing mechanisms.

Highlights

Superconducting circuits are widely recognized as a powerful potential platform for quantum computation and stand at the frontier of quantum error correction.[1]
Circuit elements with degenerate phase states that only allow tunneling of pairs of Cooper pairs, meaning their potential energy is U 1⁄4 ÀEJ cos 2φ, have been developed in recent years as a building block for topologically protected qubits.[11,12]
This effect arises because the operator φ induces transitions between the Cooper pair parity manifolds, as can be seen from the Fourier series for Eq (6)

Summary

INTRODUCTION

Superconducting circuits are widely recognized as a powerful potential platform for quantum computation and stand at the frontier of quantum error correction.[1]. In this case, the wavefunctions are localized near φ 1⁄4 0; π (see Fig. 1d), resulting in a nearly degenerate harmonic level arrangement. A brief discussion about the concept of protection and examples of protected qubits, as well as our perspectives on readout and control, follows

RESULTS

DISCUSSION