Cross entropy benchmarking functionality does not match documentation

Documentation: https://github.com/quantumlib/Cirq/blob/1d35492f85aab580807709b3b012adeeda86cba7/cirq/experiments/cross_entropy_benchmarking.py#L107 Code: https://github.com/quantumlib/Cirq/blob/1d35492f85aab580807709b3b012adeeda86cba7/cirq/experiments/cross_entropy_benchmarking.py#L335-L337

In the code, f_meas seems to be a quite different thing from what’s documented: it is obtained by estimating the experimental bitstring probabilities prob_meas through a experimental histogram (note that most of these will be 0 at a modest number of qubits), and then multiplying that number by the theoretical probability.

It appears that the documentation is based on the assumption of exponentially distributed (Porter-Thomas) probabilities, while the code is doing something else. Is that right?

@XiaoMiQC

[mpharrigan]

for your second comment @viathor there’s https://github.com/quantumlib/Cirq/issues/3775

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