Counting statistics in multiple path geometries and the fluctuations of the integrated current in a quantum stirring device

The amount Q of particles that are transported via a path of motion is characterized by its expectation value < Q > and by its variance Var(Q). We analyze what happens if a particle has two optional paths available to get from one site to another site, and in particular what is Var(Q) for the current which is induced in a quantum stirring device. It turns out that coherent splitting and the stirring effect are intimately related and cannot be understood within the framework of the prevailing probabilistic theory.