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## Noise in the helical edge channel anisotropically coupled to a local spin

We calculate the frequency-dependent shot noise in the edge states of a two-dimensional topological insulator coupled to a magnetic impurity with spin *S* = 1/2 of arbitrary anisotropy. If the anisotropy is absent, the noise is purely thermal at low frequencies, but tends to the Poissonian noise of the full current *I* at high frequencies. If the interaction only flips the impurity spin but conserves those of electrons, the noise at high voltages *eV* ≫ *T* is frequency-independent. Both the noise and the backscattering current *I**bs* saturate at voltage-independent values. Finally, if the Hamiltonian contains all types of non-spin-conserving scattering, the noise at high voltages becomes frequency-dependent again. At low frequencies, its ratio to 2*eI**bs*is larger than 1 and may reach 2 in the limit *I**bs* → 0. At high frequencies it tends to 1.