  Scaling up to a large number of qubits with high-precision control is
essential in the demonstrations of quantum computational advantage to
exponentially outpace the classical hardware and algorithmic improvements.
Here, we develop a two-dimensional programmable superconducting quantum
processor, \textit{Zuchongzhi}, which is composed of 66 functional qubits in a
tunable coupling architecture. To characterize the performance of the whole
system, we perform random quantum circuits sampling for benchmarking, up to a
system size of 56 qubits and 20 cycles. The computational cost of the classical
simulation of this task is estimated to be 2-3 orders of magnitude higher than
the previous work on 53-qubit Sycamore processor [Nature \textbf{574}, 505
(2019)]. We estimate that the sampling task finished by \textit{Zuchongzhi} in
about 1.2 hours will take the most powerful supercomputer at least 8 years. Our
work establishes an unambiguous quantum computational advantage that is
infeasible for classical computation in a reasonable amount of time. The
high-precision and programmable quantum computing platform opens a new door to
explore novel many-body phenomena and implement complex quantum algorithms.
