解决 Sycamore 量子优越性电路的采样问题 Solving the Sampling Problem of the Sycamore Quantum Supremacy Circuits

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 impossible and has been used to demonstrate quantum supremacy. We propose a new method to classically solve this problem by contracting the corresponding tensor network just once, which is massively more efficient than existing methods in obtaining a large number of uncorrelated samples with a target fidelity. For the Sycamore quantum circuits with 53 qubits and 20 cycles, we have generated 1 million uncorrelated bitstrings that are sampled from a distribution with the approximate state having fidelity of roughly 0.0037. The whole computation has cost about 15 hours with 512 V100 GPUs. 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.