  For reaction-diffusion processes with at most bimolecular reactants, we
derive well-behaved, numerically tractable, exact Langevin equations that
govern a stochastic variable related to the response field in field theory.
Using duality relations, we show how the particle number and other quantities
of interest can be computed. Our work clarifies long-standing conceptual issues
encountered in field-theoretical approaches and paves the way for systematic
numerical and theoretical analyses of reaction-diffusion problems.
