Verifying Multipartite Entangled GHZ States via Multiple Quantum
  Coherences
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.
Ken X. Wei Isaac Lauer Srikanth Srinivasan Neereja Sundaresan Douglas T. McClure David Toyli David C. McKay Jay M. Gambetta Sarah Sheldon