Tunable coupling between three qubits as a building block for a superconducting quantum computer

TL;DR

This paper extends flux qubit coupling schemes to a three-qubit system with tunable interactions, analyzing geometries and optimizing two-qubit gates for scalable superconducting quantum computers.

Contribution

It introduces a three-qubit coupling architecture with tunable interactions and optimized gate sequences, advancing scalable superconducting quantum computing.

Findings

Interaction strength depends on coupler dimensions.

Effective on/off coupling states achieved with geometrical control.

Optimized pulse sequences for high-fidelity two-qubit gates.

Abstract

Large scale quantum computers will consist of many interacting qubits. In this paper we expand the two flux qubit coupling scheme first devised in [Phys. Rev. B {\bf 70}, 140501 (2004)] and realized in [Science {\bf 314}, 1427 (2006)] to a three-qubit, two-coupler scenario. We study L-shaped and line-shaped coupler geometries, and show how the interaction strength between qubits changes in terms of the couplers' dimensions. We explore two cases: the "on-state" where the interaction energy between two nearest-neighbor qubits is high, and the "off-state" where it is turned off. In both situations we study the undesirable crosstalk with the third qubit. Finally, we use the GRAPE algorithm to find efficient pulse sequences for two-qubit gates subject to our calculated physical constraints on the coupling strength.