Controllable coupling of superconducting flux qubits

TL;DR

This paper demonstrates a method to controllably tune the coupling strength between two superconducting flux qubits using a specially designed coupler loop with three Josephson junctions, enabling in situ adjustment of quantum interactions.

Contribution

The authors introduce a novel coupler design with three Josephson junctions that allows in situ tunable coupling between flux qubits, including the ability to switch between ferromagnetic and antiferromagnetic coupling.

Findings

Coupling strength J was tunable from ~45 mK to -55 mK.

J could be tuned to zero at an intermediate coupler bias.

Measurements on a second sample linked two-qubit coupling to three-qubit behavior.

Abstract

We have realized controllable coupling between two three-junction flux qubits by inserting an additional coupler loop between them, containing three Josephson junctions. Two of these are shared with the qubit loops, providing strong qubit--coupler interaction. The third junction gives the coupler a nontrivial current--flux relation; its derivative (i.e., the susceptibility) determines the coupling strength J, which thus is tunable in situ via the coupler's flux bias. In the qubit regime, J was varied from ~45 (antiferromagnetic) to ~ -55 mK (ferromagnetic); in particular, J vanishes for an intermediate coupler bias. Measurements on a second sample illuminate the relation between two-qubit tunable coupling and three-qubit behavior.