Generation and verification of 27-qubit Greenberger-Horne-Zeilinger states in a superconducting quantum computer

TL;DR

This paper reports the generation and verification of 27-qubit GHZ states on a superconducting quantum computer, demonstrating genuine multipartite entanglement with high fidelity and benchmarking error mitigation techniques.

Contribution

It presents the first measurement of 27-qubit GHZ states on a superconducting device, combining error mitigation and verification methods to improve fidelity and confirm GME.

Findings

Achieved 0.546 fidelity for 27-qubit GHZ state.

Demonstrated GME across 27 qubits with 98.6% confidence.

Showed modest but detectable improvement from parity verification.

Abstract

Generating and detecting genuine multipartite entanglement (GME) of sizeable quantum states prepared on physical devices is an important benchmark for highlighting the progress of near-term quantum computers. A common approach to certify GME is to prepare a Greenberger-Horne-Zeilinger (GHZ) state and measure a GHZ fidelity of at least 0.5. We measure the fidelities using multiple quantum coherences of GHZ states on 11 to 27 qubits prepared on the IBM Quantum ibmq_montreal device. Combinations of quantum readout error mitigation (QREM) and parity verification error detection are applied to the states. A fidelity of $0.546 \pm 0.017$ was recorded for a 27-qubit GHZ state when QREM was used, demonstrating GME across the full device with a confidence level of 98.6%. We benchmarked the effect of parity verification on GHZ fidelity for two GHZ state preparation embeddings on the heavy-hexagon architecture. The results show that the effect of parity verification, while relatively modest, led to a detectable improvement of GHZ fidelity.