Full counting statistics in fermionic systems often involves interference of multiple particle-hole excitations close to a Fermi surface. This type of multi-particle
interference also emerges in other physical and mathematical contexts, including Fermi-edge singularity, orthogonality catastrophe and Fisher-Hartwig conjecture for Toeplitz determinants. A remarkable feature of this problem is the contributions of multiple sectors of states differing by the total number of particles. In this talk, I briefly overview earlier works on full counting statistics relevant for this problem and apply those insights to recent Ramsey-interference experiments on cold fermionic atoms.