We present a set of practical benchmarks for N-qubit arrays that economically test the fidelity of achieving multi-qubit nonclassicality. The benchmarks are measurable correlators similar to two-qubit Bell correlators, and are derived from a particular set of geometric structures from the N-qubit Pauli group. These structures prove the Greenberger–Horne–Zeilinger (GHZ) theorem, while the derived correlators witness genuine N-partite entanglement and establish a tight lower bound on the fidelity of particular stabilizer state preparations. The correlators need only M ≤ N + 1 distinct measurement settings, as opposed to the 22N − 1 settings that would normally be required to tomographically verify their associated stabilizer states. We optimize the measurements of these correlators for a physical array of qubits that can be nearest-neighbor-coupled using a circuit of controlled-Z gates with constant gate depth to form N-qubit linear cluster states. We numerically simulate the provided circuits for a realistic scenario with N = 3, …, 9 qubits, using ranges of T1 energy relaxation times, T2 dephasing times, and controlled-Z gate-fidelities consistent with Google’s 9-qubit superconducting chip. The simulations verify the tightness of the fidelity bounds and witness nonclassicality for all nine qubits, while also showing ample room for improvement in chip performance.
Waegell, M. & Dressel, J. Benchmarks of nonclassicality for qubit arrays. npj Quantum Inf. 5, 66 (2019). https://doi.org/10.1038/s41534-019-0181-8
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