  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$\pm$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.
