A few months ago the group of Jian-Wei Pan at USTC reported a random circuit sampling experiment with 56 qubits, slightly more than the original 53 qubit experiment by the Google team. Theorists have been working on ways to crack the quantum supremacy by predicting probability distribution of bitstrings using classical computers, proposing clever techniques including partitioning the full circuit into tractable sub-circuits, using GPUs to efficiently sample correlated bitstrings, and tensor network-based simulations, reducing the required classical simulation time from thousands of years to days.
The exponential scaling of Hilbert space means that a small increase in the number of qubits can increase the classical computational burden by orders of magnitude. In this vein, today the USTC team released a preprint reporting an upgraded 66 qubit quantum processor, increasing the difficulty of classical simulation by three orders of magnitude compared to their earlier preprint. In addition to the higher qubit count, the readout fidelity is improved enabling sampling from deeper random circuits.
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