[tagrun]

- Cross-entropy is used for roughly estimating the fidelity (see arXiv:1608.00263), which is independent of the problem they're solving. You're confusing it with the overall distribution of the readout states, which is the hard problem.

- CZ is a perfect entangler. No, iSWAP and CZ do not form a universal set. They typically combine virtual Z rotations (which has a continuous angle parameter) with an additional one-qubit gate, such as a X or Y gate (or their square root, which they mention in the paper) which gives a universal set. Although they don't make use of continuous Z rotations in that particular experiment, the device is capable of doing that, and is a (noisy) universal quantum computer.