Generating multipartite nonlocality to benchmark quantum computers

We show that quantum computers can be used for producing large n-partite nonlocality, thereby providing a method to benchmark them. The main challenges to overcome are as follows: (i) The interaction topology might not allow arbitrary two-qubit gates. (ii) Noise limits the Bell violation. (iii) The number of combinations of local measurements grows exponentially with n. To overcome (i), we point out that graph states that are compatible with the two-qubit connectivity of the computer can be efficiently prepared. To mitigate (ii), we note that for specific graph states, there are n-partite Bell inequalities whose resistance to white noise increases exponentially with n. To address (iii) for any n and any connectivity, we introduce an estimator that relies on random sampling. As a result, we propose a method for producing n-partite Bell nonlocality with unprecedented large n. This allows one, in return, to benchmark nonclassical correlations regardless of the number of qubits or the connectivity. We test our approach by using a simulation for a noisy IBM quantum computer, which predicts n-partite Bell nonlocality for at least n=24 qubits.