Shot noise in Aharonov-Bohm interferometers: Comparison of helical and conventional setups

We study the shot noise of current through the edge states of a two-dimensional topological insulator placed in magnetic field and compare the obtained results with the shot noise in conventional single-channel spinless Aharonov-Bohm interferometer. We find general formulas for the Fano factors of these setups, assuming that temperature exceeds level spacing in the system. We demonstrate that both in helical and in conventional case the interference effects dramatically change the Fano factor and its magnetic field dependence. For weak tunneling coupling with leads, the Fano factors of both setups exhibit a periodic series of sharp Aharonov-Bohm peaks with variation of magnetic flux piercing the system. Our key finding is that the Fano factor in the helical interferometer provides information about the presence of backscattering defects violating topological protection. In particular, the amplitude of Aharonov-Bohm peaks in the helical setup is proportional to the strength of the defect in contrast to conventional setup, where peaks have finite amplitude even in the ballistic case.