The ability to generate and verify multipartite entanglement is an important benchmark for near-term quantum devices devices. We develop a scalable entanglement metric based on multiple quantum coherences, and demonstrate experimentally on a 20-qubit superconducting device - the IBM Q System One. We report a state fidelity of 0.5165±0.0036 for an 18-qubit GHZ state, indicating multipartite entanglement across all 18 qubits. Our entanglement metric is robust to noise and only requires measuring the population in the ground state; it can be readily applied to other quantum devices to verify multipartite entanglement.
@article{arxiv.1905.05720,
title = {Verifying Multipartite Entangled GHZ States via Multiple Quantum Coherences},
author = {Ken X. Wei and Isaac Lauer and Srikanth Srinivasan and Neereja Sundaresan and Douglas T. McClure and David Toyli and David C. McKay and Jay M. Gambetta and Sarah Sheldon},
journal= {arXiv preprint arXiv:1905.05720},
year = {2020}
}