  Gaussian boson sampling is a promising model for demonstrating quantum
computational supremacy, which eases the experimental challenge of the standard
boson-sampling proposal. Here by analyzing the computational costs of classical
simulation of Gaussian boson sampling,we establish a lower bound for achieving
quantum computational supremacy for a class of Gaussian boson-sampling
problems, where squeezed states are injected into every input mode.
Specifically, we propose a method for simplifying the brute-force calculations
for the transition probabilities in Gaussian boson sampling, leading to a
significant reduction of the simulation costs. Particularly, our numerical
results indicate that we can simulate 18 photons Gaussian boson sampling at the
output subspace on a normal laptop, 20 photons on a commercial workstation with
256 cores, and suggest about 30 photons for supercomputers. These numbers are
significantly smaller than those in standard boson sampling, suggesting
Gaussian boson sampling may be more feasible for demonstrating quantum
computational supremacy.
