Abstract
We study the problem of generating independent samples from the output distribution of Google's Sycamore quantum circuits with a target fidelity, which is believed to be beyond the reach of classical supercomputers and has been used to demonstrate quantum supremacy. We propose a method to classically solve this problem by contracting the corresponding tensor network just once, and is massively more efficient than existing methods in generating a large number of uncorrelated samples with a target fidelity. For the Sycamore quantum supremacy circuit with 53 qubits and 20 cycles, we have generated 1 x 10(6) uncorrelated bitstrings s which are sampled from a distribution (P) over cap (s) = vertical bar(psi) over cap (s)vertical bar(2), where the approximate state (psi) over cap has fidelity F approximate to 0.0037. The whole computation has cost about 15 h on a computational cluster with 512 GPUs. The obtained 1 x 10(6) samples, the contraction code and contraction order are made public. If our algorithm could be implemented with high efficiency on a modern supercomputer with ExaFLOPS performance, we estimate that ideally, the simulation would cost a few dozens of seconds, which is faster than Google's quantum hardware.