Generation of Three-Qubit Entangled States using Superconducting Phase Qubits

TL;DR

This paper reports the experimental creation and measurement of genuine three-qubit entangled states, specifically GHZ and W states, using superconducting phase qubits, advancing quantum information processing capabilities.

Contribution

The authors demonstrate the generation and full characterization of GHZ and W states in a superconducting qubit system, showing their entanglement properties and non-separability.

Findings

Successfully created GHZ and W states in superconducting qubits

States satisfy entanglement witnesses confirming genuine three-qubit entanglement

States are fully characterized using quantum state tomography

Abstract

Entanglement is one of the key resources required for quantum computation, so experimentally creating and measuring entangled states is of crucial importance in the various physical implementations of a quantum computer. In superconducting qubits, two-qubit entangled states have been demonstrated and used to show violations of Bell's Inequality and to implement simple quantum algorithms. Unlike the two-qubit case, however, where all maximally-entangled two-qubit states are equivalent up to local changes of basis, three qubits can be entangled in two fundamentally different ways, typified by the states $|\mathrm{GHZ}> = (|000> + |111>)/\sqrt{2}$ and $|\mathrm{W}> = (|001> + |010> + |100>)/\sqrt{3}$. Here we demonstrate the operation of three coupled superconducting phase qubits and use them to create and measure $|\mathrm{GHZ}>$ and $|\mathrm{W}>$ states. The states are fully characterized using quantum state tomography and are shown to satisfy entanglement witnesses, confirming that they are indeed examples of three-qubit entanglement and are not separable into mixtures of two-qubit entanglement.