Fast simulations of continuous-variable circuits using the coherent state decomposition

We present \texttt{lcg\_plus}, an open-source Python library for the simulation of continuous-variable quantum circuits with both generaldyne and photon-number-resolving detector capabilities. Our framework merges the linear combination of Gaussians methodology with the coherent state decomposition of arbitrary non-Gaussian states, forming a bridge between the Gaussian and Fock basis representations. By tracking the Wigner function, we can simulate the action of Gaussian channels and measurements on multi-mode systems in a fast and accurate numerical framework. The calculation of the quality measures of quantum states is convenient in this formalism, and we derive expressions for the analytical gradients of these measures with respect to parameterized circuit elements. We demonstrate the utility of this methodology by optimizing the heralded preparation of a qunaught state, a crucial component for building a fault-tolerant photonic quantum computer, with a Gaussian Boson sampling circuit containing inefficient components.