I've been working on Google quantum supremacy paper for quite some time now and I have a problem in understanding how exactly they simulate their actual random quantum circuit on a classical computer.
To be specific, in their quantum random circuit each cycle includes a single qubit gate and a two-qubit gate. The single-qubit gate is chosen randomly from a set of three gates. To randomize the procedure of choosing the single-qubit gate there is a pseudorandom number generator which is initialized with a seed according to Supplementary information of the paper (part VII).
So my question is, when they want to run the simulation of the circuit on a classical computer, how exactly do they choose the single-qubit gate randomly?
Do they use a separate pseudorandom number generator for their code? If so, how do they compare the simulation results to the actual circuit, because the two are not related at all? Or they use the same pseudorandom number generator? Or none of them and they don't choose the single qubits randomly in the simulation but they look at the physical circuit and trace the gates that have been used in that circuit and then try to perform the exact set of gates on the exact same qubits in their simulation?
Please if you know where I can find the actual code of the simulation, mention that too. Thank you for your attention!