  We introduce JuliVQC: a light-weight, yet extremely efficient variational
quantum circuit simulator. JuliVQC is part of an effort for classical
simulation of the \textit{Zuchongzhi} quantum processors, where it is
extensively used to characterize the circuit noises, as a building block in the
Schr$\ddot{\text{o}}$dinger-Feynman algorithm for classical verification and
performance benchmarking, and for variational optimization of the Fsim gate
parameters. The design principle of JuliVQC is three-fold: (1) Transparent
implementation of its core algorithms, realized by using the high-performance
script language Julia; (2) Efficiency is the focus, with a cache-friendly
implementation of each elementary operations and support for shared-memory
parallelization; (3) Native support of automatic differentiation for both the
noiseless and noisy quantum circuits. We perform extensive numerical
experiments on JuliVQC in different application scenarios, including quantum
circuits, variational quantum circuits and their noisy counterparts, which show
that its performance is among the top of the popular alternatives.
