  In this work, we present a unified gas-kinetic particle (UGKP) method for the
simulation of multiscale photon transport. The multiscale nature of the
particle method mainly comes from the recovery of the time evolution flux
function in the unified gas-kinetic scheme (UGKS) through a coupled dynamic
process of particle transport and collision. This practice improves the
original operator splitting approach in the Monte Carlo method, such as the
separated treatment of particle transport and collision. As a result, with the
variation of the ratio between numerical time step and local photon's collision
time, different transport physics can be fully captured in a single
computation. In the diffusive limit, the UGKP method could recover the solution
of the diffusion equation with the cell size and time step being much larger
than the photon's mean free path and the mean collision time. In the free
transport limit, it presents an exact particle tracking process as the original
Monte Carlo method. In the transition regime, the weights of particle free
transport and collision are determined by the ratio of local numerical time
step to the photon's collision time. Several one-dimensional numerical examples
covering all transport regimes from the optically thin to optically thick are
computed to validate the accuracy and efficiency of the current scheme. In
comparison with the $S_N$ discrete ordinate method, the UGKP method is based on
particles and avoids the discretization of particle velocity space, which does
not suffer from the ray effect.
