Four-qubit device with mixed couplings

TL;DR

This paper reports the first experimental implementation of a four-qubit superconducting device with mixed ferro- and antiferromagnetic couplings, demonstrating ground state behavior consistent with quantum predictions.

Contribution

It introduces a novel four-qubit circuit with mixed couplings and provides experimental validation of its quantum ground state properties.

Findings

Successful fabrication of a four-qubit device with mixed couplings

Ground state response matches quantum-mechanical predictions

First experimental demonstration of such a multi-qubit system

Abstract

We present the first experimental results on a device with more than two superconducting qubits. The circuit consists of four three-junction flux qubits, with simultaneous ferro- and antiferromagnetic coupling implemented using shared Josephson junctions. Its response, which is dominated by the ground state, is characterized using low-frequency impedance measurement with a superconducting tank circuit coupled to the qubits. The results are found to be in excellent agreement with the quantum-mechanical predictions.