Generation and verification of 27-qubit Greenberger-Horne-Zeilinger
  states in a superconducting quantum computer
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.
Gary J. Mooney Gregory A. L. White Charles D. Hill Lloyd C. L. Hollenberg