An efficient Monte Carlo scheme for Zakai equations

In this paper we develop a numerical method for efficiently approximating solutions of certain Zakai equations in high dimensions. The key idea is to transform a given Zakai SPDE into a PDE with random coefficients. We show that under suitable regularity assumptions on the coefficients of the Zakai equation the corresponding random PDE admits a solution random field which, conditionally on the random coefficients, can be written as a classical solution of a second order linear parabolic PDE. This makes it possible to apply the Feynman--Kac formula to obtain an efficient Monte Carlo scheme for computing approximate solutions of Zakai equations. The approach achieves good results in up to 100 dimensions with fast run times.