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I was going through the Google's 2019 paper & had difficulties regarding some details.

How are they calculating XEB in quantum supremacy regime? To calculate the XEB,we also need the ideal distribution(simulated on a classical computer). But, that is not available in supremacy regime (out of reach for classical simulation). Are elided or patch circuit simulation possible at supremacy regime?

Page 508 Fig 3

As a follow-on of the previous question,in supremacy regime, there are 2 timescales given (one in red & one in grey). They mentioned,

To estimate the classical computational cost of the supremacy circuits (grey numbers in Fig. 4b), we ran portions of the quantum circuit simu- lation on both the Summit supercomputer as well as on Google clusters and extrapolated to the full cost.

So, grey numbers are scaled from shorter simulations taking into account experimental fidelity. Then what are the red numbers? They noted,

We expect that the full data for Fig. 4b should have similar fidelities, but since the simulation times (red numbers) take too long to check, we have archived the data

2)Does that mean the red numbers are full circuit simulation (not elided or patch)? Was the required timescale also derived by scaling from classically verifiable region? What is the difference between red & grey numbers?

Thank you in advance.

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