  We propose a general tensor network method for simulating quantum circuits.
The method is massively more efficient in computing a large number of
correlated bitstring amplitudes and probabilities than existing methods. As an
application, we study the sampling problem of Google's Sycamore circuits, which
are believed to be beyond the reach of classical supercomputers and have been
used to demonstrate quantum supremacy. Using our method, employing a small
computational cluster containing 60 graphical processing units (GPUs), we have
generated one million correlated bitstrings with some entries fixed, from the
Sycamore circuit with 53 qubits and 20 cycles, with linear cross-entropy
benchmark (XEB) fidelity equals 0.739, which is much higher than those in
Google's quantum supremacy experiments.
